Yes, this is what I was going to post.
Most of the companies say somewhere on their website that they rent for use by someone over the age of 18.
It is also in the rental agreement that renters sign - along with agreeing to only have the adult driver - no passengers.
Many people probably don't actually read the rental contract they are signing, so could say they don't know.
But, if there is an accident, damage or injury, the contract could be voided and the rente could be liable for damages, as Wheeled Traveler mentioned.
A lot of people look at a wheelchair as a horrible thing - close to a prison sentence, using words like "wheelchair bound" and "confined to a wheelchair". That's just silly - people don't use words like that about things like glasses. I'm not "glasses bound" or "confined to glasses."
People rightly see glasses as a tool that helps people do things they could not do without them.
My yougest DD is literally bound to her wheelchair -
- she has a seatbelt (a special one that pulls down on her hips),
- straps to keep her feet in place (so her abnormal muscles don't pull them out of place)
- and her seatback comes up high on the sides under her arms (to support her so sitting for long periods is not fatiguing)
But, even with all that, her wheelchair is not confining, it is freedom. Without it, she can't do the things she wants to do.
Hers is bright pink, with custom (mom made) seat covers in colors of her choice, with glow in the dark stars on the tire rims. Because it is hers, it is blinged out to her taste.
But, I have seen people at WDW who decorated their rental wheelchairs. We add solar powered fairy lights to DD's chair - attaching them with black electrical tape makes them easy to remove later. Some people put signs; I saw an ECV once with a laminated sign attached that said "The Kicking Cancer's Butt Tour".
So, if you think people might look, choose what you want them to look at.