Well, so many others posted long, detailed posts that there really isn't much that I can add. I mean, I'm 28, though sometimes I feel like I'm going on either 17 or 57.
I say that because it illustrates the point that you are only as old as you allow yourself to feel. On some days, I feel old when I know I'm not, and I'm doing better at dealing with those days. XD
On most days though, I feel young at heart still, and I would prefer to remain like that. I hold a job, pay the car payment, contribute to insurance, pay my own life insurance, pay the water bill, etc., though I still live with my parents.
In the end, you only feel old if you mentally accept it. Granted yeah, there will be physical challenges, but some have been able to beat them.