In boys clothing what age would a size 26/28 fit?

You're talking about UK sizes right? Well if so then it would fit a 5-6 year old depending on build. Maybe even 7 if they are a small build. Better getting them to try it on first.
Depends on the guy's weight. 26/28 is pretty small, though, so either a really skinny guy or a younger pre-teen boy.
my son is 4 and his new school jumper is a 26
10-14 approx . , but it could vary 5yrs either side .

Thats a waist size . do u mean chest ?
one up
It totally depends on the 'build' of the child, best to try things on!
my daddy wears 32/34
so im not very sure
In America - age 1 to 2 years! :-)