Post-pubertal maturation

Tanner G5 is not the end of male maturation. From the late teens through the mid-twenties, the body continues to fill in its adult phenotype, and the brain is still completing its remodeling.

What is still happening

By the time a young man reaches G5 (typically age 14–18), the reproductive maturation is complete: adult genital size, adult hormone levels, full fertility. But several other developmental processes are still in progress.

Continued androgenic effects

The androgenic effects of testosterone and DHT in tissues with slow remodeling continue for years:

• Facial hair — mustache, then chin, then cheeks, then full beard. Often not complete until the early-to-mid 20s, and some men continue to add density into their 30s.
• Body hair — chest, abdomen, back, sometimes shoulders. Continues filling in throughout the 20s.
• Temporal hair recession — begins in some men in their late teens and 20s under DHT influence on genetically susceptible follicles; precursor to male-pattern baldness in those who develop it.
• Skin — sebaceous glands stabilize at adult activity; acne typically wanes from its G3–G4 peak.

Body composition and bone

• Muscle mass — continues to develop, peaking typically in the late 20s.
• Bone mineral density — accrual continues for several years after height stops increasing; peak bone mass is reached in the late 20s to early 30s. This matters across the rest of the lifespan: a higher peak bone mass means more reserve before age-related loss.
• Jaw widening — adult facial proportions, including a wider mandible and brow ridge, continue to develop into the 20s.

Brain maturation

The brain is still maturing throughout this period. Notable trajectories:

• Prefrontal cortex — the planning, impulse-control, and risk-assessment region — completes its myelination and synaptic pruning last, often not finishing until the mid-20s.
• Limbic–prefrontal balance — earlier in adolescence, the limbic (emotional) system is mature while the prefrontal (regulatory) system is not, which contributes to the characteristic adolescent profile of high reward-seeking with weaker impulse control. The gap closes through the 20s.
• Myelination — overall white-matter development continues into the third decade.

Hormonal stabilization

• Testosterone — reaches its lifetime peak somewhere in the late teens to early 20s, then begins a very gradual decline (~1% per year from the third decade onward).
• HPG pulse pattern — stabilizes at the adult ~90-minute interval.
• SHBG — rises modestly through young adulthood, modulating the free (bioavailable) testosterone fraction.

When is “fully adult”?

There is no single endpoint. By the rough scoreboard:

• Sexual maturation — complete by G5 (mid-to-late teens).
• Adult height — reached at G5, when growth plates close.
• Adult facial and body hair — typically by mid-20s.
• Peak muscle mass — late 20s.
• Peak bone mass — late 20s to early 30s.
• Brain maturation (prefrontal) — mid-20s.

The mid-20s convention for “biologically adult male” reflects this: it is when the slowest of these trajectories have caught up.

Key idea

The visible drama of puberty is concentrated in roughly five years, but the program it sets in motion takes another decade to fully play out. A 25-year-old man has a different brain, different muscle mass, different facial features, and different bone density than the same person at 18 — driven by the same hormonal program, just running on the slow tissues.