Help! why am i feeling a little 'old' at 31?!?

hi there. i have just recently seperated from my abusive husband, and am feeling pretty good. this weekend, i went shopping and bought a fab dress. it is quite short though, especially on me as i am 5'8" tall. i felt a million dollars in it, even though it does show alot of flesh! trouble is, i went to work this morning and found out that my 21 year old colleague has bought the exact same dress. she didn't seem too overjoyed about it, probably cos im 10 years older than her! my question is: is 31 too old to be dressing up in short dresses and skirts, or am i just feeling a little insecure? at the end of the day, i am single again, and i do love partying and going out! thanks in advance xx