This forum is dead, but the scene is alive and kicking. Most people I know are aged around 18-28, but there are younger people training... unfortunately I don't know them well. The people I meet up with are more advanced, not really suitable for a complete beginner. Generally, training in Liverpool is just arranged between groups of friends over facebook on an ad hoc basis.
You could try asking http://airbornentertainment.co.uk/
what they offer for that age group in terms of gym sessions. I'm sure they may be able to point you in the right direction. Otherwise pm me and I can try to suggest some local younger facebook contacts.