Michelle Forbes. Provided she plays a human or perhaps a Vulcan. She was pretty badass as Admiral Cain on BSG, and since she's currently 45 she'd be the perfect age to play a captain. Kirk at 30-something was a bit young, even though he looked a bit older than his real age. Realistically one'd expect a career of at least 15 years in Starfleet before getting a shot at the captain's rank, probably more than that.
Yes I remember doing research on US Naval Captains and XO's which included ages in order to get an idea of the basic range for characters. And if I recall , Captains of Cruisers ranged in age from 44-51 with the average being 46/47. They averaged over 20 years of service, not inc their time in the Academy, for those that did attend it.
I think only Shatner was noticeably below that. Mulgrew was slightly below (40 I think). Lets just overlook Chris Pine. I dont want to get into that again. But I do think they should stay somewhere close to this range.