At 19 you should be far from 'done growing'. You're in your late teens meaning your body is still full of natural hormones that'll help you grow. You don't need M1T or 1-AD etc for the time being. Put them in a cupbaord or something and come back to them in a few years. Besides, if you're not growing now, you're not likely to grow on this stuff either. You need a diet to support it, and since that diet isn't seeing you make any gains naturally, then it's not up to scratch and won't do anything with drugs.
Now you say you may eat a lot (which is a good start) but are you eating the right stuff? Remember, you need to consume 1/1.5g of protein per lb (decent whole proteins from fish, eggs, chicken, steak and shakes etc - not the proteins people tend to count in bread-based foods) of bodweight each day, and add carb calories until you start to grow with minimal fat gain (although some fat gain comes with bulking). Nail your diet, train hard and you'll see decent gains.
I started training about 18, and although I trimmed up and was extremely defined - I had no mass. Even at 19, I'd got stronger and a bit bigger, but not the sort that made me look like a bodybuilder - I looked more like a gymnast or some scrawny Thai fighter (not a bad look by any means, just not the one I wanted). Anyway, to cut a long story short, I only started putting on size when I went back to the drawing board with my diet, which was in dire need of a rethink. Trust me, get a great diet, and you'll get great gains. Also, if you've made a foundation of muscle with regular lifting, try to incorporate new lifts and styles of training in order to shock and stress the muscles into new realms of development and growth. Do this, and your body will respond like it's on these drugs anyway.
I did M1T last year, and it did give me some decent gains but, again, I was on a reliable diet and one that I had 'boosted' to compensate for the heavier training and new growth. M1T is not a bad choice in general, but a bad choice for now. 4 months of it - especially on 20mg e/d - will melt your liver too.
Lastly, if you do take it, and you experience no ill-health, then great. However, you will be stumping you overall growth potential. So you end up putting on 5-10 lbs, how easy do you think it'll be gaining size naturally afterwards - or even keeping the gains you've busted your ass making? You'd be better off building a great physique naturally, getting a strong, seasoned body with mature muscle (this reduces future chance of injury too). Then think about M1T/1-AD or even proper gear to make it even better. Steroids are the icing on the cake, not the cake itself.