I get your point, you want to compare strictly by 5 year age increments. I'm cool with that. You could omit some 26 year old Kenyans if you knew who they were? ;)
You ask "why they are doing so well?" To answer, I watched some great runners come back after missions and I'd say there is an advantage, but its not at all physical. Two years/0-30 miles total, (vs. 2 years/7200-9000 miles total) makes for a long, painful and dangerous comeback at first.
But the maturity gain also makes them humble, hard-working and patient enough to be very coachable. Add good coaching to the mix and you can explain the success.
I saw 6,8 even 12 months for some just to get back to where they were before they left. Lots of injuries and LOTS of time cross training.
Only took me 4, but that's cause I was fat and slow before I left. :)
An anonymous description of Henry Marsh 6 months off his mission:
"He wasn't worth the bullet to shoot him." (said with a smile, of course)