It also depends on what aspects of the martial arts you are considering. Unless consistent training and focus has been maintained (and often even with this), for tournament/competition purposes regarding contact fighting/kumite, by late thirties, or at a push early forties, you are getting beyond your peak with respect to overall endurance/strength/power. Even more importantly, from a recovery perspective after heavy fights, you do not seem to recover or shrug off injuries as easily or as quickly as when you were younger.
Then again, the above is a generalisation and you do get folks such as George Forman putting the gloves back on after twenty years on the bench and handing out a whoopin and holding their own at the top level. But I would say the recovery aspects as to recuperating from injuries still stand.
Out of the ring, where you are not fettered by certain rules which negate some techniques and call for prolonged endurance, it’s a different matter. In my late mid-late twenties when I would consider myself at my all-around peak on a fight-fit perspective with high skill levels and not bad timing, I could not beat my sensei (older, not as strong, not as fit) as his timing was down perfect. His intuition and ability to read my “wa” (intentions) - basically he had “seen it all” - coupled with his timing meant he would beat me. Also, even after lots of speed work, his hand speed was always easily as good as mine.
Again, out of the ring (and outside actual physical or cognitive frailty setting in) as a martial artist in general I am not sure if an age can be put on it; it is very hard to say where age and experience is finally overcome by less experience combined with better strength/fitness.
In my gut I would say that, out of the ring and for “street”/”reality” purposes a practitioner who has trained diligently and intelligently since youth in conjunction with ongoing conditioning training would be at their peak (being a mixture of strength, speed, fitness, skill, timing, awareness, experience) at mid/late thirties and should be able to overcome those younger due to their strength and experience and also outfight those older due to their physical condition. A generalisation again, but that’s where my money would go.