There's a useful article that was posted on HN recently about how metabolism doesn't change from years 20-60. Except for legitimate health conditions, the only reason for weight change from 21 years old is lifestyle and diet. Hence this thread's original topic is entirely plausible: there's no biological reason one shouldn't wear the same jeans one wore when they were 21. In your case it also holds true: it doesn't take extreme athleticism to be lean, just a healthy and moderate diet. That may not seem normal today where obesity is becoming an everyday thing, but the average weight today is what's unhealthy, not being able to wear the same jeans you wore at 21.

https://www.bbc.com/news/health-58186710