And this other study comes to the opposite conclusion:
"People are least happy in their 40s and early 50s. They reach a nadir at a global average of 46".
Obviously, I have no first-hand experience to speak from, but I can't help but think that there's likely to be some systematic bias involved in asking exclusively people over 65. I'm not claiming I know specifically what it is, but it seems overwhelmingly likely. It may have something to do with their 40s being the most recent previous "era" of their life; they're so far removed from the person they were when they were 20 and 30 that it's just not fathomable to go back to that.
I don't necessarily know what it would be, but I'd take the results of the poll with a grain of salt, especially considering this contradictory link.
As someone nearing that middle, I think a lot of it is which direction you look. Was high school your peak, is this moment the peak, or will next year be the peak? And what do you mean by peak? Physical, mental, spiritual, social?
The points he brings up applies well to 'older' startup founders. It's hard to quantify but that extra life experience does make one better at negotiating, give more confidence in ones abilities, make you better at BS-detection and so on and so forth.
Additionally, you've probably passed through the various stages of being motivated by wanting to impress women, by wanting to gather materialistic wealth (and fast), by wanting to impress your peer-group, older founder is now doing it for more intrinsic reasons which makes for sustainable long-term motivation and commitment.
I say this as a 30 y.o guy who's currently learning how to code (previously a product manager at BigCorp) so I can build my own project this year. Sometimes I think to myself I wish I did this earlier but then I realise that the sum of my life experience has put me where I am now.
http://www.economist.com/node/17722567