The US is pathologically concerned with blame, and with avoiding blame. Everything about the US makes a ton more sense if you understand that.
If you don't empower workers, then anything they do is their fault. If you do, then anything they do might be your fault, for failing to restrict them ahead of time. It's as simple as that.
So you let them make as few choices as possible (then, if they do and it's bad, you're off the hook).
No, big chains like that in the US don't even empower middle management.
People are much worse at estimating ages then they think. If the policy is "ID anyone that looks <21" many 16/17 year olds will look 23 and not get asked. I worked for a chain of liquor stores that had an "ID 25 and under" policy - and to check it, they would send in 25 year olds. Lots of time staff would fail those checks. In my jurisdiction the penalties for selling to minors are strong enough that I'd consider a blanket "ID everyone" policy, or at least "ID everyone <40" or similar.
Well, not sure if typo, but the margin of error is the point surely? It's fine that 23 year olds who look 26 slip through as long as 17 year olds who look 23 don't. Otherwise the store is effectively saying "we expect your judgement to be up to 7 years too high and account for that" but also "you're in trouble if your judgement is 1 year too high"
Though the extent to which staff really do challenge anyone who looks under 25 obviously varies considerably, even in businesses where Challenge 25 is the official policy (which afaik is a recommendation that most chains, including pubs and supermarkets, and many small businesses follow, but not a legal requirement).
I cannot guess ages with anywhere near that precision. In California the usual standard is if you look under 30.