I started watching this when I was 11 years old, which, now that I look back, is very young. I think it's best for kids 13 & up, but it depends as well. If your child is mature, then I think it should be okay for 11 and 12 year olds. But if your child is grossed out by kissing, or easily gets scared from suspense and murder, then you should definitely wait to watch this show when you're around 13. There is also some language, but saying "b*tch" is as far as it goes, I think. There is occasional drinking, as well. Also, expect some kissing and maybe sometimes making out. Because it's ABC Family, they have never showed sex, but it is implied (it was implied four times).
Now, the plot is great. When I was watching season one, I finished the season in three days, and immediately got hooked on season two as well. But I feel like the show has been dragging since season three, it is going down in quality. My personal favorite liar is Spencer, because I feel like she is brave and intellectual. I think the writers have done a good job developing most characters, except for Aria, who I feel is underdeveloped as a character.
Also, that reminds me, Aria is romantically involved with a teacher named Ezra Fitz. They see each other while Aria is in his class until he goes to teach at Hollis College in season two. I don't think that is something people would want their kids to see, but if your child knows what is right from wrong, then there shouldn't be any harm.
I'd rate this show a 7.5/10.