It is rated TV-14 and the CW says that many parents think its okay for ages 14+. 14 year olds are in high school and at that point they probably already are discussing most of the subject matter in this show. They probably already see worse content on the internet and/or at school.
While, few can deny that there are much more wholesome TV shows, but simply not allowing the children to watch the show will ultimately backfire. When the parent forbids their child to watch certain shows, simply explaining that the show is "bad," the child may become curious about why it is so bad. Ultimately, they will end up watching the forbidden show, the parent will find out, the child will be scolded, for disobeying and the parent will be upset that their child deliberately disobeyed. Instead of forbidding the show, why doesn't the parent allow the child to watch the show. The parent can watch it, with their child and then explain that the show isn't showing great role models.
"Family Guy" is an adult cartoon, just like other offerings such as "American Dad" and "King of the Hill." I wouldn't let my young children watch "Family Guy" because there are other things for my kids such as Disney Channel and Nickelodeon. Kids don't need to grow up before they are ready. "Family Guy" is a funny show for many, but not for my rated-G household.