Looking at the City of San Diego Rules, here is what the above link says:
(Scooters) 21235.
The operator of a motorized scooter shall not do any of the following:
(c) Operate a motorized scooter without wearing a properly fitted and fastened bicycle helmet that meets the standards described in Section 21212.
So, finding Section 21212 is a little more difficult. But, I found this:
https://www.sandiego.gov/sites/default/files/bicycle-pedestrian-safety-brochure.pdf
Safety for Scooters and Skateboarders
Every skateboarder, and those using scooters or skates should ALWAYS wear a helmet. (It’s the law for everyone under the age of 18 years.) (VC 21212(a))
So, what I'm gathering is, it's suggested for everyone but required for those under 18.
Further down, it says this:
- Helmets are required for everyone under the age of 18 years when riding a bicycle, scooter, skateboard or skates. (VC21212 (a))
- Helmets are strongly recommended for people of all ages.
- Wearing a properly fitted helmet can reduce the risk of serious head injury by 85%
But, even FURTHER down, it says:
Don’t forget – helmets are required to scooter too! (VC21212(a))