I guess I can be categorized as a "cautious mom." My plan was to consider getting Catherine and Sabrina bicycles next summer, when they were almost 8 and almost 6. They seemed perfectly happy with their tricycles and I was afraid that bikes would be a huge degree of difficulty for them. But, Catherine's new physical therapist (well, she has worked with us for a few months) told me that both Catherine and Sabrina were ready now for bikes and that there was no reason to wait. Catherine does not do well in heat, so we decided that we would get more outdoor use of the bikes in the fall and the Atlanta winter then in the summer.
So, I went to Target and ended up with two 16 inch bicycles: one Cinderella themed and one princess themed. They love them and figured out how to ride them in less than five minutes. We have an area in our neighborhood with a cul de sac which is flat. The girls can ride around that circle forever!