Alice is the oldest and I think she was about 20 when she got married.
Eleanor is the middle girl, but she and Jacey aren't the same age because when he goes off to college, Eleanor is still in high school with Doug. So I would say she was 17.
Pam, as the youngest was probably 16 in the beginning.
Jacey, being 2 years younger than Alice, would make him 18. It also fits that he goes off to college very soon in the movie.
Doug was probably the same age as Pam, which would put him at 16 in the beginning. Plus, Jacey was 2 and the mom 8 months pregnant with Doug when the father died, so that's about right.
At the beginning ( in 1957 ) I thought pam and doug would have been 17,eleanor and jacey were 19 and alice was 21.So by the end of the film in 1960 pam and doug would have been 20,eleanor and jacey 22 and alice 24.