I think this is a really fair question, but it leads me to a few other questions.
What are you supposed to wear at what age?
I am 41, am I stuck with New Balance and cheap Target Polos now, or does that start at 51?
I mean do I really have to wear boat shoes and Dockers even though I hate them?
My stance on the band shirt: I still wear some band shirts, but they are from bands that 99.9% of people do not know about, and only bought when I actually attend a show for a band. I no longer rock the "Metallica - Metal Up Your Ass" shirt, or any of the other shirts that I wore in my late teens and twenties. Hell, I do not even have most of those shirts anymore. I am sure there are a few tucked away in a tub somewhere, the wife has a few tucked away too, but they are ones that either have some sentimental value or some real dollar value.
Generally when I am going casual I wear a plain T-shirt or a print that I find entertaining.
Do I think there is an expiration date on the heavy metal and hardcore hip hop shirts, sure. I think most people agree, except for those met at a heavy metal concert. However, there are some classic mainstream bands out there that I see shirts of and honestly do not hate if a middle aged dude is wearing them. A few that come to mind: Ramones, Nirvana, Pink Floyd, etc.