i present female, and people still think i'm ten years younger. when i was in greece, only a week ago, someone asked if my (9yo) daughter was my sister. a 50 something old guy once didn't try to hit on me at a bar, because he thought i was too young. started university (again) at nearly 28, celebrating at a bar, someone who was there when the first course opened, asked me if i was really old enough to be there. hairstyle seems to matter most.
it's pointless to correct people, it's much better to get a good laugh at the mistaken's stupefied look when they finally realize. or enjoy being taken for a kid, while you still can. it's only their fault for assuming.