I usually wait until frost kills the vine before I pick them, however, don't wait too long or frost will kill the pumpkins, also, and they'll rot rather quickly after they thaw out.

If a pumpkin is mature but not fully ripe, it can be picked and left in a 60-70F area to ripen. You can tell if a squash or pumpkin (in this case, Baby Boo) is mature is if there is no change in the size of the fruit for a week. If it is ripe, the fruit stem will start to turn brown. Also, in the case of Baby Boo, the skin will get a light yellowish sheen.

Depending upon growing conditions, Baby Boo can be very variable in size. In a short growing season, they will be small. In a long one, they can get to be larger than a basketball. If eating them, wait until they mature before picking. If ornamental only, pick anytime.