Dev role at late 50's?

I don't work on the LAMP stack so I don't know what that specific market is like. I would suggest you look at LAMP jobs on the likes of linkedin/irishjobs/indeed/monster etc. and get a feel for what the market is like and what the most common requirements are. While you've flagged OOP specifically, that may not be the issue. The breadth of requirements in IT roles (even beyond the coding elements) can be quite wide depending on the company.

If OOP is an issue, you now have a reason now to change - just because you haven't done - doesn't mean you can't. Whether it's refactoring an old project, or starting a new one try get some hands on experience so that in an interview situation you can talk about it "from experience". If there are gaps in your knowledge, there are so many resources on the internet that can help you.

Beyond OOP as I've alluded to already, the breadth of IT dev roles can be quite wide but it depends on the company. Try and find a bit of commonality, a lot of IT job specs. are just plain wankery, figure what the common core requirements are across the varies specs. If you have done it for 25 years there's a lot of stuff you do know. The trick here is to know how to talk about it as being relevant in the present tense. If a common thing is working agile/scrum for instance and you never worked that way! firstly learn what it's about, secondly look at how you used to work, as a one man shop you may have worked in an agile fashion even if it wasn't formal scrum iterations. But maybe, you iterated short cycles with your customers and tried to have some deliverable every couple of weeks to get constant feedback ;)

Is programming a young person's game. Yes and no. "Production Rate" is not a factor. The factor is ideally about being open to continual improvement (whether that is actually true in real terms), open to collaboration and open to change. Experience doesn't always teach us the best lessons, and we end up closing off from things, whether it's that we found a comfort zone, life priorities change, or just that the next new things is a fad like that old thing!

But if you were looking at BA or PM, and it's not working, maybe look at scrum training. One of my favourite agile talks :D

HTH

Of a similar vintage to your good self. Being in IT now is like living in a Dilbert cartoon. The industry has become so specialised in terms of roles that you'll find yourself doing pretty much rote and maintenance work if you're coding.

As Kenneth Williams used to say "everyone's getting better at less and less!". Most large internal IT Departments I've worked with recently are comprised of about 60%-70% not technical people. Failed middle-managers, admins and Powerpoint jockeys posing as architects or BAs.

Yes, I'm old and bitter!

I've been writing code for over 20 years and I am still developing and designing, not in any way working by rote!

OP if you are just doing it for something to do, then expect a very junior position and you might struggle to get that.

You might fare better at a contracting role, where you can choose your rates to make sure you get work, but that will be a 9-5 working on a specific, possibly boring thing.

Companies hiring junior developers typically want people who will grow into more senior roles, there is always room for some who will just churn out work, but not too many in my experience.