I always guessed, based on conversation, relative maturity level, and other small cues throughout the comic:
Sandy, Shoey, Bandit, and Swampy are mid 20s
Pirta, Jake, and Tawny are right at or just before 30
Doc, Roger, and possibly Howie are mid to late 30s
Red is approaching 40
Rainman is 18 or so
Jinx and Kasi are about 10 to 12
Kasi's Mom (who according to Fountains of Wayne has got it going on) is mid 30s