What is the big deal about a female dating a younger male?

(any input about workplace relationships is not wanted and irrelevant to this question)A girl I work with is 21 and she had been flirting with me for a while and all my friends saw it. I figured she was out of my league and/or had a boyfriend. When I found out she was really wanting a man in her life I decided to ask her out. She said no because I am only 18. She also said if I was older she would definitely say yes.