Why do some people look younger than their age?

My older sister has always been confused for someone younger. Right now she's 28 and the guy (he's 25) she went on a date with thought she was 22. They'll still going out.

When she really was 22 it was a bit harder for her to get dates without sometimes getting asked how old is she and once went to a club to get a drink, someone had to confirmed that she wasn't underaged. Never had that happened to me though.
Why do some people look younger than their age?
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