Congratulations. I think it's really important to be skinny for as long as possible. It's natural to gain as you get older (though some people do get skinnier), so you want a lower starting point. Keep at it.

I fluctuate between 155 and 165 depending on whether I'm using creatine (I can hold about 10lbs of water when I am). I'm in my mid-40s and my abs show. I put in a lot of work though, and it's not getting any easier.