Well, to be blunt, you feel that way because it's true. It sucks to go through but it's a normal part of the maturity process in life.
In many ways though, I think I've gotten better as a person with age. Definitely wiser, definitely less prone to making the same stupid mistakes I kept making through my 20s. Certain things do become more difficult, and of course physically things start to change as well but if you keep yourself in decent shape you notice it a lot less. I'm in way better shape now at 31 than I was at 25.
It's not really something to be feared though, because you can't stop it. You need to embrace it and make changes he want to see in your life, open yourself up to the possibility of rearranging your priorities and growing as a person. The longer you dwell on it the less likely you are to fully take advantage of the youth that you still have, which is quite a bit honestly, especially if you keep a healthy attitude and lifestyle.