Welcome to ScubaBoard!
This question gets asked a lot, and the answer is clearly "it depends."
There are people who will cite statistics on the abundance of instructors and low pay to try to convince you it is a mistake. On the other hand, there are clearly people who are making their living doing it, so it must be possible. I once met an instructor in the Caribbean who seemed to be doing OK, and he was a lot like you. He came out of the British army, and as a going away gift, the British army apparently trains its retiring soldiers for a vocation. He chose scuba.
IMO, if you really want to make a living in scuba, look at the entire package. By all means, be able to instruct, but expect to be much more heavily involved in all parts of an operation than that. A dive operation has employees who do all sorts of things, from cleaning the store to skippering the boats. The more you can do, the more valuable you will be.
---------- Post Merged at 05:22 PM ---------- Previous Post was at 05:16 PM ----------
Just to add to this related to age...
I was 54 when I became a divemaster, and I was an instructor about a year later. I never intended to make a living at it, though. I saw it as a means of supplementing my retirement and helping keep me in touch with a sport I loved. I also had been a career educator, so I happen to enjoy the teaching process itself. It's therefore a good fit for me, even though it will never be a real income for me. That, however, has a whole lot to do with where I live (Colorado), where the local diving scene is not much to speak of.