My dad isn't a software engineer, but more of a combination mechanical/aerospace engineer (MA in mechanical, PhD in aerospace), but his official job is a tech-lead right now, which he likes, but he misses "real" engineering".

Pretty much every evening and weekend, he's mucking around with something in FreeCAD and knee-deep in equations that I don't fully understand, because he genuinely loves this stuff, and doesn't want to ever become rusty.

He's 58 now, but I get the impression that he'll be doing this until his deathbed.