He was supposed to be 12 and she was supposed to be about 15/16, in fact doesn't she have a birthday party/gathering at some point in the film where they are going on about her finally being over the age of consent? (admittedly, the actress is probably a lot closer to 20 in real life).
Good film though. Stephen Graham (Combo) is absolutely superb in it, as is little Thomas Turgoose.