---
title: "The Stages of Life"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 15
exercises: 10
source: https://one-course.com/books/biology/1/en/chapter/15-the-stages-of-life
---

# Chapter 15 — The Stages of Life

Line up a family photograph: the baby on Grandma’s [knee](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint), the schoolchild missing two front teeth, the teenage cousin suddenly taller than his mother, the parents, Grandma herself. One family, one afternoon — and the whole human [life cycle](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) sitting on one sofa. This year we look closely at those stages, and at how growth itself can be measured.

## 15.1 The stages of a human life

**Definition 15.1 (The stages of human life).**

A human life passes through stages:

1. the *baby* (roughly the first two years): fastest growth of all, first teeth, first steps, first words;
2. the *child* : steady growth, [baby teeth](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#prop-g2-teeth-and-eating-well-sets) traded for [adult teeth](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#prop-g2-teeth-and-eating-well-sets) , learning of every kind;
3. the *adolescent* : a new burst of fast growth, and the body changing from a child’s into an adult’s;
4. the *adult* : growth in height over; adults can become parents;
5. *old age* : hair whitens, [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) wrinkles, strength slowly declines — the natural last stage of the cycle.

**Example 15.2 (Reading the sofa).**

On the family sofa: the baby (stage 1), the gap-toothed schoolchild (stage 2 — the changeover of [Chapter 12](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#ch-g2-teeth-and-eating-well) in full swing), the sixteen-year-old cousin (stage 3, mid-burst), the parents (stage 4), Grandma (stage 5). Five stages, one family.

![The whole cycle on one sofa: baby, child, adolescent, adults, grandmother.](https://one-course.com/images/onecourse/chapters/biology-1/g3-stages-of-life/img-c39e44616cea.jpg)

*The whole cycle on one sofa: baby, child, [adolescent](#def-g3-stages-of-life-stages), [adults](#def-g3-stages-of-life-stages), grandmother.*

**Remark 15.3 (Growing is not steady).**

Growth has a rhythm: very fast for the baby — height roughly half as much again in the first year — slower and steady through childhood, fast again for the [adolescent](#def-g3-stages-of-life-stages), then over. Two bursts with a calm stretch between: that is the normal shape of growing up.

## 15.2 Measuring growth

**Definition 15.4 (Growth chart).**

A *growth chart* is a record of height (or weight) measured again and again over time — the door-frame marks of [Chapter 4](https://one-course.com/books/biology/1/en/chapter/4-my-body#ch-g1-my-body), made regular and dated. Doctors keep one for every child.

![A growth chart from birth to adulthood: steep for the baby, steady for the child, steep again for the adolescent, then flat — growth is over.](https://one-course.com/images/onecourse/chapters/biology-1/g3-stages-of-life/fig-94aae3831cc0.svg)

*A [growth chart](#def-g3-stages-of-life-chart) from [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) to adulthood: steep for the baby, steady for the child, steep again for the [adolescent](#def-g3-stages-of-life-stages), then flat — growth is over.*

**Example 15.5 (Reading the chart).**

On the chart, the line climbs about $25\,\mathrm{cm}$ in the very first year — and only about $6\,\mathrm{cm}$ a year through childhood. The two steep parts are the two bursts of [Remark 15.3](#rem-g3-stages-of-life-bursts); the flat end says the [adult](#def-g3-stages-of-life-stages) has stopped growing taller.

**Method 15.6 (Keeping your own growth chart).**

1. Measure height carefully (the wall-and-book way of the earlier years), always without shoes;
2. write the date and the height in a notebook, or mark the door frame and date the mark;
3. measure again every few months — regularity matters more than frequency;
4. to see the story, plot the points: age along the bottom, height up the side, like the figure.

## 15.3 Animals and plants have stages too

**Proposition 15.7 (Stages across the living world).**

Every [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) passes through stages of its own version of the cycle — young, [adult](#def-g3-stages-of-life-stages), [old age](#def-g3-stages-of-life-stages). The young stage may look like the [adult](#def-g3-stages-of-life-stages) (the kitten, the foal) or nothing like it (the tadpole, the [caterpillar](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-butterfly) of [Chapter 8](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ch-g2-animals-grow-up)); a [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)’s stages run [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed), [seedling](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle), young [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), flowering [adult](#def-g3-stages-of-life-stages). What none of them can do is skip a stage or run them backward.

**Proof.** *Admitted at this level.* ∎

**Example 15.8 (How fast, how slow).**

The speeds differ enormously. A fly is [adult](#def-g3-stages-of-life-stages) in weeks; a cat in about a year; a human in nearly twenty years; an oak takes several tens of years before its first acorns. Slow or fast, the order of the stages never changes.

**Remark 15.9 (Old age is a stage, not a defect).**

White hair and wrinkles are not an illness: they are the body’s last stage, as natural as the baby’s first teeth. In many species, and in human families, the old ones carry what the young most need — experience.

## 15.4 Exercises

**Exercise 15.1 ★.**

Name the five stages of a human life, in order.

**Solution of Exercise 15.1.**

Baby, child, [adolescent](#def-g3-stages-of-life-stages), [adult](#def-g3-stages-of-life-stages), [old age](#def-g3-stages-of-life-stages).

**Exercise 15.2 ★.**

At which stage: (a) do the [baby teeth](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#prop-g2-teeth-and-eating-well-sets) fall out? (b) can a person become a parent? (c) is growth the fastest of the whole life?

**Solution of Exercise 15.2.**

(a) In childhood, from about age six. (b) At the [adult](#def-g3-stages-of-life-stages) stage. (c) At the baby stage — the first year is the fastest of all.

**Exercise 15.3 ★.**

What are the two bursts of growth, and what lies between them?

**Solution of Exercise 15.3.**

The baby’s burst and the [adolescent](#def-g3-stages-of-life-stages)’s burst, with the calm, steady growth of childhood between them.

**Exercise 15.4 ★.**

On the [growth chart](#def-g3-stages-of-life-chart), how can you tell from the line alone that an [adult](#def-g3-stages-of-life-stages) has stopped growing?

**Solution of Exercise 15.4.**

The line goes flat: age keeps advancing along the bottom, but the height no longer climbs.

**Exercise 15.5 ★.**

Using the chart, about how many centimetres does the line climb between ages $6$ and $8$? And in the single first year?

**Solution of Exercise 15.5.**

Between $6$ and $8$: from about $115\,\mathrm{cm}$ to about $127\,\mathrm{cm}$ — roughly $12\,\mathrm{cm}$ over the two years. In the first year alone: from about $50\,\mathrm{cm}$ to about $75\,\mathrm{cm}$ — roughly $25\,\mathrm{cm}$, twice as much as those two childhood years together.

**Exercise 15.6 ★.**

Why must height always be measured without shoes, the same way each time? What could go wrong otherwise?

**Solution of Exercise 15.6.**

Because a comparison is only fair if the measurements are made the same way. Shoes one day and socks the next, or a sloppy slanted book, would create “growth” (or hide it) that never happened.

**Exercise 15.7 ★.**

Give the stages of an oak tree’s life, from acorn to old tree.

**Solution of Exercise 15.7.**

Acorn ([seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed)), [seedling](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle), young oak, [adult](#def-g3-stages-of-life-stages) flowering-and-acorn-bearing oak, old oak — and, in time, death.

**Exercise 15.8 ★.**

Try it: start the [growth chart](#def-g3-stages-of-life-chart) of [Method 15.6](#met-g3-stages-of-life-keep) for yourself, with today’s first point. Which stage of [Definition 15.1](#def-g3-stages-of-life-stages) are you in — and which burst is still ahead of you?

**Solution of Exercise 15.8.**

Open answer. A nine-year-old is in the child stage; the [adolescent](#def-g3-stages-of-life-stages) burst is still ahead.

**Exercise 15.9 ★★.**

A fly, a cat, a human and an oak run the same stages at very different speeds. Order them from fastest to slowest to reach adulthood — and say what all four orders of stages have in common.

**Solution of Exercise 15.9.**

Fly (weeks), cat (about a year), human (nearly twenty years), oak (several tens of years). In all four, the order of stages is the same and cannot be skipped or run backward: young, [adult](#def-g3-stages-of-life-stages), [old age](#def-g3-stages-of-life-stages).

**Exercise 15.10 ★★.**

Tom, aged $9$, measures himself twice in one month and finds “no growth at all”. Should he worry? Answer with [Remark 15.3](#rem-g3-stages-of-life-bursts) and the shape of the chart at his age.

**Solution of Exercise 15.10.**

No. At nine he is in childhood’s calm stretch — about $6\,\mathrm{cm}$ a *year*, which is half a centimetre a month: far too little to see between two measurements a month apart. The chart climbs surely, but slowly; he need only wait a few months between points.