My daughter last year at five and a half.
That's an older 127 Motive package board. A smaller board would made her more comfortable getting ready but we could sure tow her slow. Idle up to 5 MPH and she's out and riding. Later we towed her at 9 MPH. "Hit it" means put it in gear and barely any throttle. The big boots did not bother her a bit.
She has a brother, 9, who rides a lot so she just assumes she's supposed to. It was way easier getting her up on the board than on skis.