You can see that it was thinning quite a bit well before then, back before he started growing it out, which I'm convinced was done solely for the purpose of hiding his baldness. All the oldest pictures I've seen of him, he has a buzzcut, which we know is not what he had as a teenager because his mom was still styling his hair back then. Then he went from short hair to just long enough to comb it back, and has not deviated from that style since then. Normally you could call "autism", but here I would argue that Phil would sport a shaved head to this day if it hadn't started falling out twelve years ago.
What's funny is that he admits to the bald spot on the back of his head, but not the one on top. He also won't wear a hat, which means he wants to live in denial. A hat would basically be admission that he's bald.