By looking at the ages of all actors as the show began or were later added to the show like Cindy and Teri who I've added their age when they came onto the show and not when the series debuted:

John Ritter (29)
Joyce DeWitt (28)
Suzanne Somers (31)
Jennilee Harrison (22)
Priscilla Barnes (26)

So oftentimes Mrs. Roper would refer the trio as the "kids" and Jack would sometimes date "older" women that he would get teased by the others and of course Jack was still going to school to be a chef.

I think the answer is yes, with the possible exception of Jennilee Harrison as Cindy.

It's interesting that Suzanne Somers was the oldest of the roommates' actors when they first premiered on the show, since Chrissy seemed to be the most immature of the original 3. And wasn't the story on Priscilla Barnes that she auditioned before the show went on the air, but gave off too much of an older vibe for the producers' liking at the time? But in real life she's actually 10 years younger than Suzanne Somers and 7-8 years younger than Ritter/DeWitt (if those numbers are correct and I'm doing the math correctly)?!

The original trio were probably all supposed to be about 25.