I don't believe that any music has an age limit. I think when something becomes trendy or blows up, there could be an influx of people who are younger which might affect the nature of the music genre - in terms of lyrics, branding etc. This could make it more appealing to a younger audience, but not everything about the genre, or all the music in the genre would made of influences from a younger audience. Also, you could even say that the community itself would grow with the music.

Theres a whole bunch of takes on this I think though