My (uncanonical) theory is that he was an officer cadet in some form of fascist/communist-style military youth group at the time of the 1st Kairopan harvest - that there was a clear career path through these groups for those who showed 'superior' qualities (young 'show ponies' for the dictatorship) and these led into fast-track officer training at a very young age. This would presumably make him at least 16 at the time, but possibly Tarrant fudged any age restrictions:
1. He may have looked older at *that* time, being tall and self-possessed and rather forward :-)
2. It's possible that the Federation is and was short of manpower at this late stage of its decay; there are other signs of this even before the Andromedan War, the heavy reliance on non-labour-intensive control methods such as drugs and planetary exiles, the poor quality of military personnel. And I doubt that there's many child-labour laws in place :-) If so, the powers that be on lesser missions like the Kairopan collections may have been willing to overlook age irregularities to get half-way decent personnel for what appears to be a pretty unpleasant mission, but which doesn't appear to take any degree of experience or skill.
Also, there's no indication that I can recall that he acted as pilot for Jarvik, just as 'lieutenant' in his first command. Given that - when he joins the Liberator - he appears inexperienced *at* command, it's possible that it was soon after the Kairopan mission (and possibly a couple of other minor missions) that he returned to FSA for his 5 years pilot's training and *didn't* act in an officer's position again.
I think I'd have to move my estimate of his age at Powerplay up therefore to 29 :-) making him 14 at the time of the 1st harvest. Yes, he still looks younger than this when we meet him, but he's just got the genes for it (odd that it doesn't seem so wrong with Deeta, who needs to be a fair few years older still); also, Tarrant does seem to me when we meet him to have a curiously untouched quality - comes across as someone who's been sheltered (as a show pony/FSA high flyer*) from the harsher realities of Federation life that Jenna, Vila etc know all too well, and it does make him think and seem younger than he is.
Sally
* this background might also explain his relatively high profile in Harvest, since high flyers would be good fodder for FSA publicity.
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Hello
Time and in effect AGE are altered by gravity and the increase and reduction of its effect on humans, as they travel through space.
Hence, any biological computation of a characters age, would have to take into account the time, distance and degree of reduced or increased gravitional effects on their cell structure.
The base point would have to be the cell aging rate on their birth planet, or system, and then adjusted for variances when they are on a planet. Then you would futher need to adjust for the time traveled in space, and the distance form any major gravational body, which is then adjusted again witht he distance form that body, and the speed at which they are traveling. ie - time distory 1, etc
Have fun !!
tony