I am 31 and still enjoy metal next to other music genres. The only difference that I can see since I was younger, is my personal outfit and apearance (clothing, hair etc.), and that also has a lot to do with the job, your development of taste, and maybe your partner. Also when you´re older you might get more tolerant (if that is the right word here) and open to other genres, that is why it might seem that people > 25 (to use your age-limit) listen to less metal. And in the end that is probably right, because you only have a certain time a day to listen to metal because of job, family and other music genres you suddenly also enjoy. But that is only pro-other things and not against metal as a genre.
I agree that getting into metal is often a thing of being cool, tough different, and a little rebell (in my case it was the Kiss masks, I had no idea about the music but wanted to have the Dynasty album because of the cover, and I got it for christmas - I was 10). That is what you want to be when you´re 10, 12 or 14. Pretty normal I would say. Comes time you maybe really start enjoying the music, but then you are more selective and don´t just buy every shit that just calls itself dark, brutal, evil or whatever, you lay more stress on the music. For me that are bands like Iron Maiden, AC/DC, Voivod, Kiss, Thin Lizzy, Accept, Dio, Scorpions etc. and from the newer bands Brainstorm. That is pretty much all I still listen to, although I have hundreds of other metal LPs and CDs. These bands I listen regularly (every day in the car) and still buy the latest albums.
Metal is not an age question, you just get more selective.
Cheerz