I think that when talking about a fighter's peak we must not only speak about physical peak but a combination of physical, and mental peak along with experience and technique.
For that reason I think 28-30 would be the right range so I picked 29. A 26 year old might have slight better reflexes than a 30 year old but most likely its not as big as a factor as the possible 4 year technique and experience difference.
Maturity can also be a major factor which many times goes hand and hand with mental stability, and from the infamous words of Timmay "90 percent of the game is half mental".
"Let them see that their words can cut you, and you'll never be free of the mockery. If they want to give you a name, take it, make it your own. Then they can't hurt you with it anymore." - Tyrion Lannister