The only reasoning that Okuda gives is that some 40 years would have to pass for the guy to be born and "be raised", that is, to reach the rank.
The bar set in ENT "First Flight" apparently features a flight patch giving the names of three people, supposedly all the participants of the mission, but no date. Browsing through Memory Alpha, I was delighted to see the old Spaceflight Chronology
referenced; the 2020 mission described there would fit the bill just as well. Hell, if pre-2020s interplanetary flight utilized cryosleep as said in "Space Seed", we could argue that Christopher launched in 2009 and arrived (either at Saturn, or back at Earth) in 2020!