As I said, its fair enough that they only really dress up for this festival (or for other goth gigs, which most of them do). I would guess that many of these middle-aged people have jobs that don't allow them to dress like that.
Then there's the fact that teenagers often wear stuff to look good in front of others, whereas adults are more likely to wear what's comfortable.
Imo, huge boots aren't comfortable at all, there's no way I'd wear them just to go shopping in. Not to mention that you see more youngsters around in the streets than adults, due to the adults actually going to pubs and stuff rather than just hanging around a park.
And the most obvious point of all, walking around dressed really over the top as an adult could/would lead to much more than just a few people saying things. As a kid, you can get away with it more easily. And finally, those kind of clothes are really expensive; they are the kind of outfits you only wear on special occasions, such as festivals like that.
Sorry, I just get really confused when people seem to think its strange that the older people who go to these festivals don't dress that way all the time. Seems like such a simple thing to understand to me.