My Favourite Sitcom Of All Time!
I'm 13 and have seen every single episode of this show and I enjoyed every moment of it! I won't give a full review of how good it is because that would take WAAAAAY too long as there is so much to say about it. So I will just give a parent's guide to this show and share my opinion.
Language: There is not much swearing in the show and it is mainly mild. Here is a list of words i remember hearing: cr*p, d*mn, h*ll, (son of a) b*tch, b****rd, wh*re and sl*t. I think that's it but if anything else is said I apologise.
Sex: The show has a fair amount of sexual innuendo but doesn't show anything graphic. There are quite a few scenes where couples are seen in bed together after sex but are under blankets so nothing can be seen. The character Howard can be a pervert in the first 4 or so seasons (talks about sex, breasts, strippers, prostitutes and stuff like that pretty much every much episode, not particularly graphic, but still perverted) but improves a lot afterwards (still makes the occasional suggestive comment but it is dialled down a lot). In one episode in Season 3, it is heavily implied that a visitor here to see Sheldon sleeps with both Leonard and Raj (This character doesn't appear again afterwards). Won't go over everything because it would take a while but generally OK for 11 and up.
Violence: Virtually none. The only few times I can think of are in Season 1 where it is implied that Sheldon's sister kicks him in the private parts (is seen emerging from his room cupping his crotch with a hunched back), in Season 3 when Penny punches Sheldon in the face when he tries to steal a piece of movie memorabilia for her and Leonard's bedroom during their sleep (he is later seen with a bruised nose), and in Season 6 when Amy accidentally hits Penny in the face with her handbag (she is also later seen with a bruised nose). As you can tell, there is nothing bad violence wise.
I think this show is great for mature 11 year olds and up. Hope I helped