i think the purely physical decline between 25-30 is completely negligible. From 30-35 that is very different for each individual, depending mostly on their genetics and how much punishment they have taken over the years. Even so the differences in speed, strength, explosiveness, endurance and reflexes between the two age groups is not a large gap... probably about a 5% decline, so skill and experience CAN more than make up for it in many cases.
so yes some fighters may be past their physical peak by a few years but might barely be entering their MMA peak... it really just depends on the fighter
Head-butting fists and KOing the air is not effective striking.