I'd be surprised if a 40+ yr old man who was relatively strong in HS could look similar in terms of body composition at the same weight (say 165 at age 18, 165 at age 40). As we age, % body fat increases naturally (assuming all other factors to be equal).

So.... I'm 44, weigh 10 pounds less now than I did as an 18 yr old, appear way less dangerous than I did as a freshman rugby player, and haven't attempted anything close to my old squat/bench/clean in 20 years. Yet I'd guess my % body fat is around 13% now, was more like 10% then (was squatting 400, benching 250 at 5' 9" 175). I lift a little, do a lot of running/cycling/climbing, and let my body composition change naturally. I'd have to restructure my life to try to get back to the muscle mass I had at age 18. Hormones matter.