What size bike for an 11 year old boy ... buying 2nd hand off ebay

[CampingManDad]

I want to buy a secondhand bike off ebay for my son who has just turned 11.
I'm a bit confused what size bike would suit him that he would get 2/3 years wear out of.
He's average height.
Some sites suggests a 24" wheel bike will suit up to 13/14 other places it suggests it's only suitable up to 11!
I'm looking at the popular brands like Specialized, Ridgeback, Dawes, Trek, Giant or Scott.
Any advice appreciated.

I don't really want to take him into a bike shop or he'll be disappointed that he's not getting a new bike!

Just got DP to check DSs and his is 26 inch wheel which is adult size. 16 inch frame which is small adult. He's a tallish 10yo, and if it's like his stepbrother, this will last him until he's about 13.

You need to get a 26" kids bike.

Probably a 26, I agree. What height is he?

He's 4 feet 10 inches (146 cm)

Then I'd go for a 26, that's what height DS was last summer when he got it

I would say 26 inch with smaller frame.
My average 7 year old is now on a 24 inch already. 5 year old on 20