When I was 15 I was convinced I was a hardgainer. Years later when I learned that I had to work consistantly with progression as my goal I was not nearly as hard a gainer. Now in my late 40's I find it fairly easy to put on muscle, even though I still have to work hard at it.
A huge advantage that I have over young people is that I don't have to fight my metabolism. I don't have to eat like a human garbage truck, to stay ahead of the curve. Working around my aging connective tissue is a downside however.