Only 2 out of 8 divisions have an average below 29 years old. The sample size for one of them (FW) is only 5, so that is obviously a huge factor. This is not "peaking", these are the ages they are when they initially win the title. It doesn't factor in how long they keep/hold the title, or what age they are when they lose the belt, only what age they were at when they initially won the title.
The sport is 30 years old at this point. The average age of champions is not going up by circumstance, it's pretty consistent that most champions are 31-33 throughout history.
21 out of 96 UFC champions (throughout history) were under the age of 27 when they won their belt. That's only 21.88%. Which means 78.12% of UFC champions throughout history have been 27 years or older.