Wait, someone accused you of being 15 because you had an Iron Maiden t-shirt on?! That makes literally no sense, Maiden are a classic 80s band. Here in the UK they have the exact opposite stigma, where people might joke about you being an old fogey.
This (but I'm in the United States)
My stepbrother (who was in eighth grade at the time) was wearing his Iron Maiden tour shirt and one of his teachers said something along the lines of: "Iron Maiden!?!
Aren't you a little young to know about them?"
I suppose that could be taken either way in terms of insults/compliments, but it serves the purpose of demonstrating that old 80s metal is generally associated with old(er) people (at least to his teacher, and I suppose people that aren't that in touch with today's music world, because if they were they'd know that old metal is undergoing a huge resurgence especially with the music video games booming).
Also, I went with my stepdad to the recording studio with his band and the studio manager guy was there and saw me with my Dream Theater tour shirt, and said: "Nice shirt! How old are you? Wow, a sixteen-year old in a Dream Theater shirt, that's what I love to see!"
So yeah, although I'm not even sixteen (two more months
), I've never gotten told that I'm listening to immature music (although being pretty much only 16, there's not really a level I can descend to age-wise), and if a comment is made, it's always pointing in the older direction.