Hello
Though of course Steven Pacey was almost fifteen years younger than Tarrant as originally written for season three, it has never bothered me. Compared to various Hollywood productions demanding the audience to believe that two people with six-year age difference are father and daughter or that teenagers really are like these people who look like they could already have teenage children of their own, it takes very little suspension of disbelief to think that Steven Pacey of 1979-81 vintage is suppose to be over 30. As for the mental side, I don't see why a man in his mid-thirties couldn't have a Flash Gordon complex just like a man in his early-twenties.
If you need an in-series excuse, plastic surgery could really be viable, as a means of repairing job-related damage; in case of Space Command officer, anything from enemy fire to having the ship's coffee maker explode in your face (any shipyard putting together ships that can't fly without wobbling through a bit of time-distort turbulence is unlikely to maintain very high standards of safety and functionality with the non-essential systems) or being told by your commanding officer to "just go check what those leathery, egg-like things in that ship wreck are, I'm sure there's no danger..." Not everyone considers battle scars to be a batch of honour or an individualist fashion statement, and when it's time to reconstruct the face, well, of course what the doctor uses is the old archive image taken from when you were just a wet-eared wanna-be star killer.
This explanation of course doesn't count for Deeta, but since I've rationalising this far, why not consider them brothers in this respect as well. Imagine Deeta, a handgun to cleaned and a way too many intoxicants to remember about basic things like checking whether the gun is loaded before you start messing it with clumsy fingers. Then imagine a lot of screaming, some embarrassing questions like "where's my nose" and then a call to the nearest plastic surgeon to get your mug to look like the official fan portrait again of the champion of Teal, and since the pictures retouched anyway, could you just snip a bit here and tighten things a bit here and...
Kai