Adulthood

The long plateau of the lifespan — the body has finished growing and maturing, and is at or near peak function.

What is happening

• Peak and plateau — most physical capacities (strength, bone density, lung capacity, fertility) peak in the twenties and then decline slowly and gradually.
• Maintenance over growth — the body’s work shifts from building itself to maintaining and repairing itself: replacing worn cells, healing injuries, holding systems in balance.
• Stability with slow change — changes are real but gradual — a slow loss of muscle mass, gradual stiffening of tissues, subtle hormonal shifts.

Why it matters

Adulthood is the longest stage and the one in which lifestyle compounds. Diet, exercise, sleep, stress, and exposures act over decades, and largely shape how the next stage — aging — unfolds.