Personally, I have had a lot easier time making gains after 25 than I did before. I worked out in high school quite a bit, and off and on after that, but my diet wasn't so great. I was bigger than the average guy, but that was mostly genetics. When I was 26, I hit the gym seriously and put on 40 lbs in 6 months, naturally. I don't think I would have had an easy time doing that at 18.
Even if natural test/GH/whatever levels are higher between 15 and 25 or whatever, I don't think the body is primed to store mass at that age nearly as well as it is after 25. Look at the pros. If natural hormone levels played that much into it, the top guys would be in their mid 20's. Instead, with a few exceptions, all of the top competing pro's are in their 30's.