How does average penis size vary by age group in American men?

1. Puberty is the main driver — growth timing and where it stops

Most sources agree penile growth happens primarily during puberty, beginning with early puberty (roughly ages 9–14) and with the fastest changes between about 12–16 years; several explain there’s no single cut‑off but that “adult size” is usually reached sometime in the mid‑ to late‑teens to early twenties — the American Academy of Pediatrics is cited as saying genital development usually finishes between about 13 and 18 years (Parents) and other health outlets say growth tends to stabilize by around age 21 ^{[1] [2]}.

2. Adult averages reported, but not broken down by fine age groups in these sources

Multiple medical and popular outlets summarize adult mean erect lengths in the same general band — roughly 5.1 to 5.5 inches (12.9–13.9 cm) — but these figures are presented as overall adult averages, not as separate means for, say, men in their 20s versus men in their 40s (WorldPopulationReview; Medical News Today) ^{[4] [3]}. The available items do not offer a systematic, age‑stratified dataset for American men by decade or narrow age bins.

3. Consensus: size stabilizes after adolescence; some say age does not shrink penis length

A commonly reported conclusion is that once adult size is reached, penis length remains largely stable; Wikipedia notes that “age is not believed to negatively correlate with penis size,” and other pieces repeat that size is generally stable into adulthood and early middle age ^{[5] [6]}. If you are looking for claims that average erect length decreases with age, those are not supported in the sources provided — available sources do not mention a consistent, population‑level decline in length with normal aging ^{[5] [6]}.

4. Variation in reported age milestones and why numbers differ

Different outlets use slightly different benchmarks: some emphasize age of onset and rapid growth (starting ~10–14), others cite when growth finishes (13–18 or up to 21), and yet others present general adult ranges without pinpointing an endpoint ^{[2] [1] [6]}. Methodological differences drive variation: self‑measured surveys tend to give higher averages than clinician‑measured studies, and many summaries synthesize international studies rather than a single U.S. cohort ^{[5] [3]}.

5. Factors that can alter development or apparent size during the life course

Reporting highlights that genetics, nutrition, hormonal health, and certain medical conditions (e.g., micropenis diagnoses, Klinefelter’s) influence final size; childhood malnutrition or endocrine disruptors can affect genital development, and adult conditions such as Peyronie’s disease may change shape or function though not necessarily average length across the population ^{[2] [1] [7]}. These are presented as modifiers, not as evidence that normal aging reduces mean penis length ^{[2] [1] [7]}.

6. What the current sources do not provide — key data gaps

None of the given sources supply a U.S. national, age‑stratified table reporting average flaccid and erect lengths by decade (teens, 20s, 30s, 40s, 50s, 60s) for American men. If you want that specific breakdown, it is “not found in current reporting” among the supplied items ^{[4] [3] [5]}.

7. How to get a more precise answer and what to watch for

To produce the age‑by‑age chart you asked about, researchers typically need clinician‑measured anthropometric data from a representative sample (or large, carefully controlled studies). When evaluating future reports, check whether measurements were self‑reported or clinician‑measured (self‑reports inflate averages), the sample’s country and age distribution, and whether erect or stretched/flaccid length is reported — those choices materially affect reported averages ^{[5] [3]}.

Bottom line: existing coverage agrees that penis growth is concentrated in puberty and usually stabilizes by late adolescence/early adulthood (about ages 16–21 in many sources), adult average erect lengths are commonly reported near 5.1–5.5 inches, and the supplied material does not provide a detailed, age‑stratified breakdown for American men across adult decades ^{[1] [2] [3] [4]}.