No, I think it's due to maturity. I think even average guys who don't train or participate in sports are generally stronger at 28 than 18. Obviously, it depends on circumstances also. If someone lifts weights all thru high school and the becomes totally inactive in their 20s, they may not be stronger at 28. But I do believe the male body continues to mature and become stronger into the 20s naturally. Resilience and ability to recover may be better at 18, but that is only 1 aspect of one's "physical peak ."