Curious if it's possible instead of going by age to legislate something like a maximum bike speed for sidewalks that would capture both young kids and adults on the final block headed to the bike rack. Like a maximum of a brisk walking pace or 8 mph or something.
LA has something like "as long as you aren't endangering pedestrians" in their ordinance allowing bikes on sidewalks. Yeah it's vague but it seems to work ok.
Ideally if there is no safe bike infrastructure (physical barrier) you must be able to bike on the sidewalk. There can be speed limit for riding on the sidewalk.