It's hard to guess what age they might reasonably choose. Both Washington and Jefferson where close to 60 when they became President. John Adams was 62. So, they may not have wanted to exclude those possibilities (seeing themselves that they had the potential.
So, perhaps they say that you can't be over 65 or 68 when inaugurated. That was pretty old then so might be considered reasonable. However, it would have ruled out Benjamin Franklin (and others) so, they might have gone higher -- to 72 or even 80, which would be largely moot.
If they picked 62, they'd rule out Harrison, Taylor, Buchanan, Ike, Reagan, and Bush 1, several 2nd terms, the last term of FDR, and MANY challengers (Hillary, McCain, Dole, etc.)
If they DO pick 62, what mechanism, if any, do they put in place to change the max over time since the lively life span continues to increase? It would make sense for it to automatically change in relation to average lifespan, but Congress has never shown an inclination for such changes to happen automatically.
Finally, I think if they were of the mindset such that they DID impose a maximum age, they might also have raised the minimum age, with the idea that there's an ideal "age range" -- perhaps 45-60.