Scoby -absolutely not too old at 45. There's nothing hard in it, mostly water, gravity, and air, then what happens if things get pressurised, , whether by accident or on purpose.
If you looked through a couple of collegy (ie NVQ2) textbooks, and looked in some catalogues to see what bits are available, you could start earning(slowly) very soon. I'd say that to anyone who's practical, bright and interested to learn, with a bit of respect for regulations which are there for good reason.
Once you're calling yourself "a plumber" you can do the Corgi thing which currently takes about 2 weeks and £1500 or so.
The hard bit comes with apparently simple things where there's stuff you didn't know you needed to know about. Recent posting illustrate, but take even a tap washer - you find you can't isolate the tap. It could be from a combi, the mains or a tank. You might consider freezing IF there's access. Turn a gate valve and it might come away in your hand, or not close, or not reopen. You might get a difficult air lock in the pipes if you drain down. You might have to work in dirty dark unboarded eaves to get to something. If it's council property everything might be locked, or shared with others. Plenty more along that road. Then you get the tap apart ( you may not be able to, even with a puller to get the handle off) to find that cos its been leaking for ages (13 years in a tap I had recently) the brass seat has eroded away. You try to recut it but it's too far gone. You can go buy plastic insets - but then there's hardly any flow at the tap when it's open - which you don't find until you've spent however long clearing that air lock. And the customer then notices that the gland seal also leaks so wants that done - sometimes not possible so they want a new tap - which may mean rebuilding a bathroom practically from scratch.
