Isn't part of the issue that early on in a start-up, everyone is indispensable, and often that comes with putting in extra hours and going the "extra mile" for the viability of the company?
When you are over 35, 30 even, you are much more likely to have a spouse, children, etc., and all these command more of your time that the business wants to capitalize on.
Personally, I've seen some real rock-star engineers both at 20 and at 40, however more often than not for different reasons. Honestly, not once have I ever learned a decent life lesson from engineers around my age (I'de say I'm on the younger side), however it's been several highly memorable times that an engineer on the older side has pulled me away, given me some real hard-hitting, incredibly valuable engineering and life lessons, and changed my career and sometimes life for the better.
It's a complex world!