To reinforce your point, here is the Youtube channel of a database admin (by day) in Tulsa, OK who spends his free time learning how to (and building) a ~70 foot steel cutter vessel in his front yard.

https://www.youtube.com/user/submarineboat

Or this http://makezine.com/2015/04/20/understand-1700-mechanical-li...

A retired mechanical engineer spends his time making videos of mechanical linkages in CAD. He's was at 1700 a year ago, hard to say how many he's at now, he's still releasing them. This is what technological progress looks like I think, in a couple of years this will be shown in just about any intro class into the subject and it will reduce the learning time by like, i don't even know, 100x? 1000x? Idk how these were described before but boy do they seem hard to describe in anything but animation.

I'm also having a hard time imagining his brain isn't getting exercised just fine.

I'm not a mechanical engineer and I spent like good 15-20 minutes watching them and getting quite a bit out of it.

S/V Seeker! I love it when two different interests of my own find a match somewhere. I've spent way too much time catching up on his videos, but he's phase inspired me in some of my own projects.

Wow, thanks so much for the link. Amazing watching people build that together.

In my view staying physically healthy is vital. From what I've seen of folks who are in their 70s and 80s, mental and physical activity are each necessary for the other, and have to be habits long before retirement is even on the horizon.