Hah! 25!!!
I am 26, sir, going on about 4, so I understand your dilemma...
However, two things come to mind. One is that humans are living way the hell longer than ever before, and there's no way to accurately predict life expectancy by the time we're old. Average U.S. life expectancy when my grandparents were born was around 65, or so, I think, and it's now something like 72 for men and 79 for women - but that gets skewed by a large number of people who get taken out young by alcoholism, drug abuse, or just general rough living, so if one makes it to 60, life expectancy within that subset goes way up, I believe. My grandparents are all in their 80s, and three of them are very, very healthy. It's not unreasonable to believe that by the time we all get to be very, very old, medicinal advances will have put the life expectancy up in the 90s, I believe.
The second is that this long life expectancy, I think, tends to create a longer period of "adolescence" in general for people. Most people now don't enter the workforce and become "adults" in a societal-economic sense (taxpayers, producers, etc.) until their early or mid-twenties. (I think this is a valid point even past my own ability to use it as a rationalization about how immature I still am, even at the age of 26.) So if you don't feel emotionally as old as you think you ought to be, there may be an actual reason for that which is not your fault in any way.
If you just want to whine about how you're not a teenager anymore, I'll get out the case in which I store the world's smallest violin...
Life's too short to feel old, anyway, even if one is!