I often feel the same way. But in certain aspects, I feel older. It's an odd combination, really.
I'm 22 years old, yet I have the wants, needs, cravings of a 14 year old (approximately) and I believe my emotional maturity is at that point as well. I still get upset over not getting my way, losing at games, and feel extremely shy, almost to the point of being afraid, of girls. I generally shrug off responsibilities, play video games all day, and carelessly spend my money without concern for the next day.
In other ways, I feel older than I am. I find very little in common with people my age, and usually identify with the opinions and customs of older people. I lack the energy and ambition that most people my age have, and no longer feel that "immortality" that most seem to believe they have until old age. I find politics, philosophy, theology, and history interesting. When I'm invited to go to a party or club, which is seldom but it does happen, I feel out of place and old. I feel like I'm not connected to the "scene" that people my age are a part of and I lack the link to their world to make me feel a part of it. I'm always tired, lethargic, and rarely possess the initiative to want to do anything active or "cool".
I dunno, I think I understand where you're coming from. I guess it's that my heart feels young, but my mind feels old. My body, as a result of this, feels restrained by itself to not embrace the age it is physically.