Am I depressed or is this just part of getting older?

I'm 21, just for reference. I just think back to 2 or 3 years ago. I wasn't as "mature" then, certainly, but I felt more "alive". Its not that life was great back then, I can think of plenty of problems and shortcomings, but I had dreams, ambitions, visions, and I was in love. Admittedly, some of all that was pretty unrealistic, even childish. But just gradually over the past few years I feel like I've been worn down. I think I'm a better/more responsible person in a lot of ways, but I just feel like I have no identity anymore. I'm going through the motions well enough, but I just don't care that much about it, about myself, or anything. I've tried to get some of that old spirit back, but I don't think its going to happen. Is this just part of getting older?