I do strongly feel it is an 'age' thing - they are clever little individuals at around the age of 7-8yrs. They have gained loads of confidence at school and are testing their boundaries at home. They have to be good at school, do as they are told etc etc and I think they like to let off steam when they get home and push us parents to our limit.
I think my 9yr old dd is coming out the other end of it now - I have so many chats with her and told her that unless she bucks her ideas up she is out of this house - I explain to her that she simply 'lives' here - and that she is lucky to do so.
Kids can be cruel, disrespectful, horrible and generally a pain in the arse. I refuse to let my dd's get to me although they obviously have in the past. I try to walk away and, as I've said before, go and count to ten in another room. If you let your kids see they get to you and wind you up they will do it all the more.
Food and fluids plays a major part in a child's behaviour. I'm certain processed foods contribute towards this. Burger Kings, McDonalds, KFC are so full of additives they can send a kid up the wall along with fizzy drinks etc. I think the same applies, to a certain degree, to chicken nuggets, smiley faces, waffles etc - remember the Jamie Oliver school dinners programmes? He fed a group of kids on the usual chicken nuggets meal for lunch and a group on good quality home cooked food - the difference to each group during the afternoon was amazing - the chicken nuggets group couldn't concentrate/remain alert - the others were fine.
Drinking water/fluids is a must - most kids spend 6hrs per day in a stuffy poorly ventilated classroom with a lot of other kids - they can easily dehydrate and, for example, forgets to drink because she's 'too busy'. Give them loads to drink and they pick up. I force my dd to drink her water immediately after school if she's not drunk it during the day.