For what it is worth, I am glad we have them. We have 4 kids so I get a range of ages on it depending on what seating we have available.
If I only had 1 (maybe 2) I'd probably not bother. The caveat would be if they seemed like they would be slow to learn to ride or less athletic riders. Meaning, you'd have an older child on the TAB with some frequency rather than on their own bike.
Good luck!
