I think there are a number of reasons this happens - ranging from overt dishonesty to an attempt to accurately represent.
Some ladies are too lazy to update their ads, or don't even know old ads are still running. The information may have been accurate when originally posted, but years have passed since. I had this happen to me a few months ago. An old avert from when I was 21 was getting top placement in a new directory. Since I have not changed my name, website url, email address or phone number since then, I ended up getting a lot of calls about it - including people asking why my website had me listed more than 10 years older than my advert. It was because I had nothing to do with that ad being posted.
Some ladies post younger because they think it will bring in more clients. Most post as younger because everybody else does, and it oddly provides a more accurate representation of someones age. Let's say a woman posts as 40. Most guys are going to look at that and assume she is in her mid to late 40's, because almost all women, including those not in the industry, tweak their age a bit. If she posted her actual age, most men would add onto that, and actually have an inaccurate idea in their heads.
Some ladies post as OLDER than they actually are. I quickly learned to do this when I started. I know there is a huge demand for 'barely legal' and college girls.....but the reality is that the majority of the clients I got when I was that young were quite dreadful. I started off posting as 19, and within a month went to 21, and by the end of the year I was posting as and stayed at 25......for several years ...... until that was accurate.
In the end, as long as a lady is of legal age, and looks like her photos, the number shouldn't be too important. Have a conversation with her on the phone to make sure you get along, and then have fun!