Working professionally with PLCs

Yes,
That’s the thing I have a full time job and would like to work part time, if possible, at the weekends.

No one advertises their need for part-time or on-again, off-again supplemental help. That doesn't mean the need isn't there.

I know several "retired" controls engineers who fill in staffing gaps for companies on a contract basis. But to find those gigs, you need contacts within the company -- HR knows nothing about them, nor does the web.

Keep in mind that for these jobs, YOU are the one that needs to be flexible. "Part-time" to the company may mean "40 hours a week for the next two weeks only", not "10 hours a week for the next two months". The man-hours are the same, but the deadline isn't.

So I should wait a few decades before hitting up gigs like this , I don't think any youngsters are doing the part-time work.

Not necessarily. I've known one or two "youngsters" who were able to swing 80+ hr weeks, working for two companies, or took vacation time to do a short-term gig.

But they were open and honest with their main employer about it. They got approval beforehand and made sure there was no conflict-of-interest.

There are worse ways to earn a couple of grand, help someone in a pinch, and learn a little something new.