I think the disconnect is that because things like greying hair and receding hairlines vary so wildly, they really aren't a true indicator of how old a person is. It's not--okay you've hit 30 and you are going to start to grey now, so with a wizard it's 40. Even if wizards age differently...it still doesn't mean they can't get their grey hair early or whathaveyou, just like Muggles.

But it could be that maybe the aging doesn't actually change, they just have longer lives. Maybe instead of magic slowing their aging/keeping them youthful for longer, it's more that magic keeps their body functioning and healthy for a longer time so that if you die a natural death, it's going to on average be at a higher age than their Muggle counterpoints.