It seems the American Heritage® Dictionary of the English Language thinks scooters are just for kids:
scoot·er(sktr)
n.
1. A child's vehicle consisting of a long footboard between two small end wheels, controlled by an upright steering handle attached to the front wheel.
2. A motor scooter.
3. A flatbottom sailboat with runners that can skim over water or ice.
While the dictionary people don't think it necessary to ascribe an age to those folks who travel via motor scooter or an ice-running sailboat (how many people hear "scooter" and actually think "ice-running sailboat"?), somehow the scooter that is propelled by a kicking human pilot is "a child's vehicle".
Allow me to get all curmudgeonly and call "balderdash!" on that (hey, I'm over 50, I've earned curmudgeon colors). Look at these folks:
They are members of the NYC Kick Scooters Club. Happy scooter owners. Happy scooter riders. But not kids.