I've never directly lied about my age, but if someone assumes I am younger than I am, and give me a child's admission price, then I don't let on.
Yeah; it didn't happen often though because usually they ask my age, and I won't lie if they ask. I used to be the other way round actually. If I wanted to buy a 15/18 DVD/game, they would think I was too young, and ask for my ID. On one occasion, I didn't have it, so couldn't buy it. Both of these things were quite a while ago now, but I still sometimes get asked about whether I need an adult or child ticket.
I've had the same problem. Once I was shopping during the day and had a shop assistant ask me why I wasn't in school. I was 22. Well into my twenties I had to show ID to buy alcohol.
That's the problem if I ever want to buy alcohol as a present for someone.