Poster 6 got it.
You know the studies. After age 30, you lose about 10% of your physical abilities every 10 years, and it starts to accelerate after age 60. That's life, you can live with it, or not.
Don't matter how much your train, lift, run, or workout.
Now some will say "this 65 year old can..."... but that guy never did anything athletic as a youth. He saved his body so he has something left.
Possibly you didn't.
In other words, if you can serve at 100 mph now, you could have served at 130 back when you were 25.