A very small percentage of people age much slower than average. Most of them are on a slower biological clock, so they mature more slowly too, and have a lower peak. For example Anthony Whiteman, 40-44 mile WR holder, was running 3:36 in his 20's. He was never a top elite, but he's much less far off his peak than most people. And of course Ed Whitlock, who wasn't a great elite but is nearer his peak than any other 80+ year old in history.
I think Lagat could run about 3:34, 8 seconds off his PR. That would put him around where Coghlan and Walker were at his age. A slow ager, but no Anthony Whiteman.
Because 3:26 is so fast, it might be suspicious if he could still run 3:30. Compare Jim Ryun who peaked at 20 and was done in a couple more years. And poor Alan Webb who can't even claw his way back to PR+14. Athletes who hit those extreme high levels early burn their metabolic candles at both ends to get there, so they don't stay that fast for long.
Why aren't there more ageless athletes? Because athetics only support the best, the ones who hit their peaks early. When they have peaked, they decline quickly and lose interest in competing. Whereas those who peak late are never good enough to make it in the first place, and when they finally peak there is no incentive any more. As little money as there is in athletics, there's diddly squat in master's athletics.