Determining when it will be outgrown will also depend on the geometry of your bike, namely: (1) how much knee room your child has based on your handlebar shape and the seat-mounting height; and (2) how far you have to bend over to reach your handlebars (such that your child's head would interfere with you bending over. Part of the reason I've been able to keep my son in the Bobike Mini until 4 is that I have North Road style handlebars that curve out to give him more knee room and curve back toward me for easy reach. If you have flat bars and a threadless headset, your child may be hitting her knees by 3. The front seat is definitely more fun though, even if it doesn't have as much longevity.