At what age should a boxer retire?

It depends on how much the fighter took care of himself inside the ring, as well as outside the ring. Look at guys like De La Hoya, it seemed like he would be around for a long time. It turned out the partying took it's toll on his body. The guy was washed up by the age of 34. You have guys who kept it smart in both areas, and they fought darn near until the age of 50. (Hopkins, Foreman)

there is no set age, some age much faster than others, it has something to do with how many fights they have and if they take a beating in the ring.

Some champions have fought for a long time and some fighters are used up in their late 20's, there is no arbitrary age for retirement.

Ask Bernard Hopkins

When theyes run out of gas

(in the voice of Samuel L Jackson) "Whenever he Gotdamn well feels like it." LOL

Too many variables. It all depends