29-year old Joanne Woodward...
...was way too old-looking to play a 17-year old. She always had a very mature face and looked like Caddy's contemporary, not her daughter.
They should have used a younger actress like Tuesday Weld or Sandra Dee.
If you have access to high school yearbooks from the 40's and 50's -- look at the senior class photos. All the girls look older than 17-18. Maybe it was the hairstyles. Back then, we tried to look like our mothers, and we succeeded.