Like you said unless you hatched them out yourself or the breeder took accurate records you won't know the exact age. Age isn't a great way to judge if they are sexually mature. All geckos grow at different rates. I have some that were all the same age when I got them, some are still only 18 grams and others are pushing 30 grams. It's all up to the individual with how much they want to eat. At this point there is no way to sex them at a young age. All you can do is wait and hope, usually around 10-15 grams you should be able to tell with a loupe. I've read over and over again that males are the hard ones to get, for whatever reason.
If you are still unsure on sexing check out JB's website here: http://jbscresties.com/sexing.html