Children aged 9 and over are much more confident speakers, and may begin swearing or using slang in order to fit in with their friends. It's best not to overreact to this, as they will often continue to do it just to wind you up!
By 12, your child will probably be able to read adult books and magazines. You may also notice that your previously chatty child becomes very quiet around you. This is normal, so don't criticise or tease your child about their behaviour and try to talk to them about things they are interested in - clothes, sports or TV - as this might be a good way to get conversation flowing.