Old enough to be mindful about the health consequences of worrying about the wrong things and not worrying about the right things.

I'm merely asking because the things I could get away in my 20's and things I can get away with in my 40's appear to be wildly different, and I don't seem to be an outlier in this respect :)

(mind you, for me second monitor is about sanity/efficiency, not ergonomics; but a well adjusted chair, keyboard and mouse make a stunning difference to my fingers & wrists these days)