your body doesn't so much break down under training, usually it breaks down because of all the other stresses in your life.. family, career, and so on.
At 48 I could outswim my 10-yr old and outrun him. At 55 and 17, he swims much faster than I ever could, and can outrun him only at half-marathon distance and over..
ran seriously in HS, college, and on. All my run PRs are from the mid-20s. This is distorted because of a bout of cerebral malaria in the late 20s which took a long time to recover from, believe I could have improved at least the longer distances in early 30s without it. I did set PRs in the short triathlon distances in my 40s, due to improving the bike speed, but never again got close to run PRs.
Body really did not start to break down until 50s. In the 40s it gets harder so you really have to think about recovery as well as training, and there's a slight slowdown 5% or less: but I had fewer injuries in my 40s than earlier decades.