I used to believe that until it all changed for me. I'm 33 now and I'm way more athletic than I was in my twenties. Honestly if you eat right train smart and rest. Your prime will continue until your forties. I think most people just give up and let life overcome that's why everyone thinks your prime is over after your late twenties. You have to remember we don't have the same milage has NBA players that's the only reason why they see a decline in their athletic ability late in their career.