Tanner stages: a shared ruler
To talk about puberty precisely, clinicians use the Tanner stages, a 1-to-5 scale describing physical maturity. Stage 1 is fully prepubertal; stage 5 is fully adult. The scale rates breast development in girls, genital development in boys, and pubic hair in both — and importantly, these three can be at slightly different stages in the same child.
Why split pubic hair from breast/genital staging? Because pubic hair is driven largely by adrenal androgens, while breast and genital growth are driven by the gonadal sex hormones. A child can show pubic hair (adrenarche) while the breasts or testes are still prepubertal — the two axes from Guide 3, written on the body.
First signs, and where the growth spurt sits
The usual first sign differs by sex. In girls it is breast budding, called thelarche, driven by rising estradiol. In boys the true first sign is enlargement of the testes, driven by FSH and LH before any obvious outward change. Knowing the correct first sign keeps you from being fooled — a boy with pubic hair but small testes has had adrenarche, not real puberty.
The growth spurt — peak growth velocity — arrives at a different point in each sex. In girls it comes early, near the start of puberty, often before their first period; menarche typically follows the peak, by which time most height is already gained. In boys it comes late, near the end, when testosterone is high. This timing difference is a big part of why adult men are, on average, taller: boys grow for longer before their spurt and add more during it.
The secular trend
Over the past century and a half, in many populations puberty has begun earlier and adult height has risen — a shift called the secular trend. Better nutrition and the control of childhood illness mean children reach the body composition that permits puberty (recall leptin from Guide 3) sooner. The trend is gradual and varies by region, but it reminds us that puberty timing is not fixed by genes alone — the environment sets the stage.