According to JHS Terra, James McKenna died aged 64 in 2338 (page 148), so that would mean he was born in 2274. Page 146 says that he became Fleet Admiral in 2295, making him at 21 years old at the time and only after having been demoted 5 times in the past decade, so he joined the Navy at 11 (which would be a year before he finished home schooling)? Even if he'd been from the Alliance's elite, and ignoring the demotion issue, being made Fleet Admiral at 21 would be extremely suspect, but he was meant to be a nobody from the backwoods of Canada.
Perhaps his age when he died needs to be tweaked a little. Given the seniority of a Fleet Admiral, maybe dying at 94 instead of 64?