I'm tired of people saying stuff like you're this age, you have more than enough testosterone in your body. That's bullshit, no you don't. You might be able to gain a little more, but not an appreciable amount. Gaining natural is so far different than gaining on AS. Even at your natural peak, it would pale in comparison to even a small cycle. I remember training when I was 19, and there was no way in hell I had plenty of testosterone in my system. Yes, you shoud up your calories, but you are likely to gain a lot of fat with that muscle. 8-10 weeks of 400-500mg test a week will do you good.