At what age does it become a compliment to be told you look younger than you are?

It seems like when girls are 21 and under they are offended if people tell them they look young for their age. After about 21 they are usually not offended. But when does it actually become a compliment? My friends cousin is 28 and was really happy when someone guessed she was about 23. I've seen females that age react in a similar way when they are told they look young for their age. I actually find this quite strange because late twenties/early 30's is still young. It's very unusual for females at that age to be past their best and show signs of ageing (unless they have taken a lot of drugs, been on sunbeds regularly etc.) The only reason I think it would be flattering is because if you look young for your age at 28, it's likely you will look young for your age when you are 50.

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