The uk cosplay community is pretty diverse and it depends what sort of events you go to what sorts of niche of people you get. For example expo has a large draw of people and therefore the cosplayers there range from the hardcore to the casual. I think it's difficult to put an age on the community for this reason. Plus the uk cosplay community isn't as young as cosplay communities in other countries so for this reason we have people at the older end of the scale
Generally though if you want to meet serious cosplayers head to the UK convention scene such as Ayacon, Amecon, Fushicon or Kitacon to name but a few. Aya and Ame are for over 18s and Fushi and Kita are for over 16. But here you will meet a wide range of people and as I said more serious about cosplay and the whole experience is a lot calmer so you will enjoy it more.
As for interest I myself am 25 and have been cosplaying for 4 years now
But I#m not buy any means an old cosplayer as a lot of my friends are older