It's retail and it's a promotion they're doing - the consequence of not asking and missing a potential "mystery customer" is having to endure patronising bull**** from higher-up jobsworths. The consequence of casting the net wide and just asking everyone who looks over 30 is ... nothing really. Potentially pissing off a customer but no front-line staff member in a large company is going to be bothered by that
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'tis the same reason the opposite happens and some 40 year olds still get ID'd for age-restricted products. It's not that the staff member is "dumb" or ACTUALLY thinks you look under-25... it's that they're playing it safe on the off change that you're an age-tester and they're going to get a bollocking for missing it and getting a fail.