Fast food joints are not safe. You can get seriously injured working a fryer or grill, and a 16 year old is less likely to be able to operate those things safely. Keep in mind also that a 16 year old is almost certainly going to school, so they will be working while already tired.
I say this as someone who started working at age 14: Children should not be allowed to work most jobs until they are much older than that.
These same arguments can be applied to other activities, like driving a car. I do support treating teenagers with more agency than children, so while I don't support letting 5 years olds work in McDonalds, I honestly don't see a problem with 16 year olds doing it.
Yes, the risk is higher than for a 25 y.o., but as someone who started working at age 15 - there are also rewards in experience, money, and figuring your own shit out. Nothing taught me about life like working.
I say this as someone who started working at age 14: Children should not be allowed to work most jobs until they are much older than that.