Indeed, they could have been 14 at the time of the prank and still posted this when they were 18. I think the person you responded to might not have fully realised their own age maybe? It’s happened to me before.

I was at a festival waiting for a band to come on. Talking to people in the audience one of them asked me if I’d seen this band before. I said “yes, in X year”. A girl there says " Wow, I was born in X year". For a second I wondered how she got the beer she was holding then quickly calculated that X year was 18 years ago and in that moment realised I was talking to a group of people nearly half my age. I hadn’t really been cognisant of it before that.