aaaw poor girl! she is gorgeous though! she will clean up to be quite a hottie. if i'd seen her at pet store i would have snatched her up in red hot second!
she is probably somewhere around a year and a half old, but like kristy said it's hard to say exactly. usually chain stores don't have adults, especially females.
do know if she was ever housed with other cresteds? often they are housed in groups and females end up gravid. since she is of good size you should be okay if that is the case, but do make sure she eats her cgd so she can keep her calcium reserves up just in case.