HOW to talk to an older girl?

Ok....so I wrk at karate center and im only 14 yrs old but many girls and ppl think im way older than that...like this woman who gave me her number today, she is about early 20 I think and she real pretty but I dnt think she know im only 14.... I teach her son who's like 6 and she just walked to me and gave me her #, how should I play it off and talk to her?????