Normally, you're only as old as you feel. If you eat processed food every day, smoke, drink, and don't exercise, you'll feel 50, when you're 25. All day long.
Then again, we had a vietnam vet with us in Iraq, in 04, and he ran 5 miles every morning. He was in just as good a shape as a lot of the 35 year olds in the unit. And this was an elite unit.
Now, if you have a freak injury, then there's nothing you can do about it. And that can happen at any time, and age does not matter. I threw my back out when I was 26 one day. Guys blow their knees out doin silly ****, so age doesn't matter at that point.
Nonetheless, you still have to eat right, get exercise, and get good sleep each night regardless. Or, you're going to feel like ****. So, if you 'feel' like 40, then that may be because of some habits you may have. I'm 36, and I'm in better shape than ANY of my friends and family, and half of my platoon mates.
I guarantee you, you'll feel a thousand times better if you change your exercise and eating program.