Mike, you should've left well enough alone to begin with. You were young and not nearly done growing. You would've grown into that weight easily.
Me, I'm still around 120 and I'm 5'9" and 23 years old. Thing is, I'm built pretty small, so even though I only weigh 120 I don't look terribly thin. Even if I am a little underweight, I'm content. Gaining weight has proven difficult for me, even on the kind of food I eat. I just have a very high metabolism. That, and I've always been very active. As quickly as I take calories in, I burn them right back off.