Because of the show’s impossible to pin down timeline, it’s hard to put an exact age and date to each image but they do leave a rough guide to follow.
Little Homer: He’s age 4, making him 31 (a 27 year gap) when his mother returns.
Young Homer: He’s age 10, same as Bart because this is a comparative flashback.
Teen Homer: He’s age 18, a college senior.
Early 20′s: 20-22, it’s been a while since college, then Marge becomes pregnant and we see the birth, so add in 9 months from the start of the episode.
Mid 20′s: 23-25, Bart is nearly 2 at the start of the episode, then Marge becomes pregnant again, add another 9 months in.
Late 20′s: 26-29, This would be a natural place to put in a frame from ‘And Maggie Makes Three’ which bridges the gap between children but Homer looks the same as usual in that episode. So here he is eating brownies with George Harrison.
Regular Homer: He could be as young as 31 or old as 38, depending on which episode you watch.
Mid 40′s: If we go with the top age, 31, and this takes place fifteen years in the future (2010 as the episode aired in 95′) that brings him to 46.
Early 70′s: Forty years have passed since the events of the episode, going with him being 31, takes him up to 71.
Of course he could be older or younger, this is simply how I’ve interpreted the information the show gives, feel free to come to your own conclusions.