My DD is turning 7 tomorrow and she will get one. Most of her peers and friends already have one, so she has been begging for a while
But these expensive kind of presents she doesn't get in between - only birthday and christmas! And even now the DS will be from her grandparents and aunts, simply because we want to make her understand the value. The DS costs here US$175 and the avarage game US$44 and I simply find that too expensive for a birthday present. Mind you, all her birthday wishes are in that price range
OK, back to your question - yes, I think your son is old enough to have one when he turnes 5 - there are age appropiate games. It's just that I think they don't need to get one at that age, kwim? But that is just my opinion, though.