Rules made for a reason
For someone who has worked at a grocery store, there are rules made for a reason.
If the cashier thinks they need to card someone, they have to do so because if not it could be their job. Personally its not worth losing someone’s job because they are doing their job.
No one is above the law. How was that cashier to know the alcohol wasn’t being bought for the 18-year-old? That’s why she carded everyone. There’s people out there that do that — buy alcohol for their kids.
No one from the public tells you how to do your job. They know how to do their job and all the rules.