Jon Jones just looked at the best as he'd ever look in his whole entire career at 35, OP, I mean, to tell you honestly, it depends on the athlete, also as we know, the maximum longevity for a fighter athlete is 13-15 years old, so if you are a fighter that fights at heavier division (cause the heavier divisions are the divisions where you can still be elite even you're old), and you started late, the longevity of your career can still be the same mostly, and it depends on if you work hard.
Like, Francis Ngannou is a fighter that can defy age, next year he will be 38 Years Old, he maybe old age wise in the sport but in his fighting years, he's still young cause 10 years is not really that old for him to really just pass his prime already as we know he started MMA at 27 years old, same with Gane who just started MMA at 28 years old, so don't expect him to pass his prime just early, maybe in the next 10 years, it just depends, Stipe started MMA professionally at 28 years old but the guy is an animal and looked good at 38 years old, same with DC who started at 30 years old, and looked really good at old age in fighting...etc, it just depends on the athlete tho.