I'd say about 27-29ish, I picked 28. If you look at the former champs and legends like Ortiz, Big Nog, Fedor, CroCop, Wanderlei, GSP etc., their peak performances and "invincible stage" seemed to be around that age.
After their early 30s, many of them were still on top but showed some slight weaknesses in their game. By the time the mid-late 30s rolled around, most of them have been toppled or pushed close to retirement.
There are some of the current champs like Aldo at 25, Bones at 24, JDS at 27 at slightly earlier than that age, but they are still growing and I believe their peak is yet to come when they hit that 27-29 ish range.