Since the show has a floating timeline we will never know their true ages but it has been hinted that:
Homer said he remembers Woodstock in 69' which puts him at around 40 years or so. But sources say he is 38.
Same goes for Marge, around Season 1 she was said to be 34, but Homer and her did meet in High School and they did go to camp together, which puts her in the same age group as Homer, which means she is between 36-40 years of age.
Bart is 10.
Lisa is 8.
Maggie should be around 1-2 years old. Marge when she was pregnant with Lisa, bought their home with Homer, it was around 7 years later when Homer got his job at the plant that they had Maggie.
