I'm 24 now and my gf is 25. Anytime we get booze at a restaurant she gets carded and I don't. Not sure if they think I look older or if it's a flatter the girl to get a bigger tip thing.
Well, it's also probably a safe bet that if she's 25, she's probably not dating a 20 year old, whereas a 25 year old guy could very well be dating a 20 year old. Saves time to just work off that assumption and card the girl first, I imagine.
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https://www.youtube.com/watch?v=wsbvaQcuW9k