Why? Why draw an arbitrary line? Is there any indication in canon that 17 is anything else than an arbitrary number picked by the magical government to define maturity? Do our abilities reach a permanent plateau in the instant we turn 18?

 

Magical power depends on how much magic a person has used and experienced. Harry has a marginal advantage due to having spent ten years of his life under some of the strongest wards in Britain, in a domestic situation that encouraged him to use lots of accidental magic.

Of course, this gives purebloods a head start, but muggle-raised wizards are much more likely to cast just for the hell of it, so it evens out.