I've a wee niece who didn't do a lick of magic until she was eight. That had my poor sister worked up to no end and half convinced she was a Squib. But when it did burst out of her, she flew both her younger brothers right down the stair and out the door for pestering her, so I reckon it was building up all that time and came out stronger for it, like a river bursting a dam, and now she's as normal as anyone.