My father is 22 years older than me. Ever since I was very early 20s, people have assumed I am his wife or partner.
For example when I was just 20 we were sat next to each other on the train, and a man on the other side said if my dad is my husband When asking to swap seats. When we have been travelling in foreign countries there have been 'lucky man' comments from locals.
I went shopping with my dad as he was going to buy me a special present for a big birthday, and the cashier also assumed I was his girlfriend!
I get very pissed off about it. Maybe this is a common problem with having youngish parents?