It is rated TV14 (in the U.S.A.) & Rated 15(in the U.K.) so obviously you should be at least 14 or 15 since it's not appropriate for people under those ages.
Obviously any age can and do watch it, it is up the parents to decide, assuming they are involved with what their children are viewing.
The Father, the Son & the Holy FonzPeter's Two DadsFamily Guy
No such character exists on Family Guy
Season 10|Episode 12 Livin' On A Prayer
Airport '07 Season 5 episode 12
Family Guy :)
Family Guy - 1999 Christmas Guy 12-8 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Mom's the Word 12-12 is rated/received certificates of: USA:TV-14 (DLSV)
Family Guy - 1999 Finders Keepers 12-1 is rated/received certificates of: USA:TV-14
Family Guy - 1999 A Fistful of Meg 12-4 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Grimm Job 12-10 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Quagmire's Quagmire 12-3 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Peter Problems 12-9 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Into Harmony's Way 12-7 is rated/received certificates of: USA:TV-14
Family Guy - 1999 To Love and Die in Dixie 3-12 is rated/received certificates of: USA:TV-14
Family Guy - 1999 3 Acts of God 12-13 is rated/received certificates of: USA:TV-14
Family Guy - 1999 Valentine's Day in Quahog 11-12 is rated/received certificates of: Canada:PG USA:TV-14
Family Guy - 1999 Airport '07 5-12 is rated/received certificates of: Australia:M Canada:13+ (Quebec) USA:M