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Peter | 43(unofficially he's 44 since he's 2 years older than Lois but his last stated age is 43) Lois | 42 Chris | 13(unofficially 14 since he's now a freshman) Meg | 18 Stewie | 1 Brian | 8
There are plenty of 'old guys' in Family Guy but the most notorious is Herbert AKA Herbert the Pervert. He is the old man that spies on Chris and is a known pedophile.
His age is not given but it is assumed he is around the same age as Peter and Lois, they are shown in school together and have similarly aged children. It is estimated that he is in his 40's.