I dont think it really matters what age he is, if he needs it, let him use it. If hes quite young and you dont want the fact he has to use something to be obvious with strong smells, try a simple, mild one that is just lightly fragranced or fragrance free.
As Psychick says I don't think it matters how old they are.
My daughter has just turned 10 and started to get a bit 'pongy'...puberty is starting earlier these days! I bought her some Dove sensitive skin roll-on to start with. It is so good I bought one for myself, as it appears to be far more effective and more gentle than the aerosols.
Do encourage him to use deoderant if he needs it. There is nothing worse than pongy armpits - I know - I have two boys! I buy my boys spray on deoderants as they can they have a spray at their feet if they are pongy aswell. Maybe you could start him off with something trendy like Lynx - but I do try and encourage my boys to use an anti-perspirant deoderant as I don't know which is worse - pongy armpits or sweaty armpits!
My son is 11 now n just started high school but last year he'd been "workin' out" as he put it (he had just watched Rocky lol)anyway he was very impressed with the fact he'd got his first smell of B.O lol he called it a man smell though! I decided to make a big deal of it and take him shopping to pick out deoderant, shower gel n hair gel n that (he bought LYNX EVERYTHING lol (purse shredded) but he wasn't embarrased at all it just made him feel really good about himself he couldn't wait to jump in the shower in the morning n get himself all spruced up! He's still like that now thank god and really takes care of himself.
Lynx smells nice but sucks. My step son used that and was always a wee bit stinky then he came back from school one day absolutly honking. I could smell his shirt in the laundry basket from the front door and it nearly knocked me out. His dad had a wee word and he uses sure now and smells lovely :) Anti perspirant has to be good, no one wants sweaty patches on their shirt!
After many years of trying different products myself, there seems to be only one really good one and that's Sure for men, but ONLY the one that is a cream, and looks like a big mushroom (ie not the spray or roll on). Its hard to find though. Tescos and superdrug stock it, but asda and sainsburys dont.