procki wrote...
I think the only clue we ever really get about her age is that video-log we can find on Pragia. If I remeber correctly the researcher in it says that now that the project has failed they will piggyback onto the Alliance Ascension Project (which I think might be the whole Gillian Grayson storyline covered in the second book).
The timeline on the wiki says that the Ascension Project was established in 2176, as Mass Effect 2 plays in 2185 this only leaves a 9 year period between her escape from Pragia and your first meeting with her. You might add a few years here, because it is quite plausible that Cerberus already knew of the planned establishment of the Ascension Project and therefore planned in advance.
After that only her age at the time of her escape remains in question. I always had the impression that she was still rather young at the time, around 10 to 12 years old. That is because even though after als her bad experiences in the facility she could still be quite easily coerced into doing things by other people, like the pirates that found her and then sold her.
edit: Now that I have written this I'm not really sure that they really talked about the Ascension Project. If they did not please disregard all this 
As long as the timeline is correct, I think your estimate on her age is probably close to correct. I don't know why, but I always got the impression that Jack was perhaps 13-15 years old when she escaped Pragia, which would put her in the 22-24 year old range when you get her out of prison.
I'm kind of curious as to the exact situation that brought Jack to Purgatory. Obviously she isn't the type of person to get caught easily if she has a chance to fight back, and she has done quite a few questionable and dangerous things in her (what is seeming more and more) short life between escaping Pragia and meeting Shepard. What happened that got her caught? Was she on another one of her dangerous, self-destructive joy rides, or did some bounty hunter just get lucky enough to catch her with her guard down and knock her out? Could be an interesting story.
And thinking back on her age, or at least her hypothesized age, how does that affect the hypothesis that some people have on her and Miranda being sisters? Now I, personally, don't necessarily believe it's true and kind of hope it's not the case, but if it is then what kind of implications does that have? Miranda is in her early 30's or late 20's, I believe, so this would mean that Jack is almost definitely younger than Miranda. It would also mean that their age difference is only 10 years at the most. I guess the first question it raises for me is that, if they are "sisters", did Jack come along before or after Miranda left her father? Was Jack perhaps the impetus for Miranda to leave (Miranda discovers what her father has done and how she came to be and decides he's evil)? It opens some interesting doors, but obviously it would all be speculation with no true base in anything we currently know about either character.