Many parents think that like Kevin, Harry Enfield's teenage creation, their children will transform from sweet 12 year olds to monster teenagers once they turn 13.
But many a mum and dad are shocked to discover that particularly their daughters will start displaying teenage-type behaviour well before adolescence.
So why does the door slamming, sarcasm, body obsession, and violent mood swings begin so early? Is it just that children are growing up quicker these days or is there a physiological reason for a change in the behaviour of pre-teens?
Author Deborah Jackson and Dr Richard Stanhope, an endocrinologist from the Portland Hospital in London join Jenni to discuss. Tell us your views/experiences For more information contact the BBC Actionline on 0800 044 044