In my experience (anecdotally, from friends and colleagues) there are three distinct phases in which the good ones are briefly single, available and date-able.
The first phase is mid-20s, by which point they're established enough in life to become partner material.
They can sort out their own bills, know the basics of how to keep a house in order, have a regular income and a stable place to live in. They also often have had enough experience with women to know the basics of how relationships work and have had enough time to make their mistakes with those and learn from them. If they find a suitable partner, that's a good chance of them being taken off the market indefinitely.
The second phase is late 30s to mid-40s.
It's usually a time when men, too, re-evaluate their life choices. It could be that they have realised they have made a mistake in choosing their first long-time partner through incompatibility or that they and their first long-time partner have simply grown apart. They now know what works in relationships and what doesn't, have shown commitment, have made their mistakes in long-term relationships. They have a career, decent income and enough assets to sustain themselves, and are able to deal with issues when they arise. They can come with children or may have been previously married, but are genuinely good men to have.
Of course, you may also find a few that have prioritised their careers over relationships, but I have observed that they often still make too many mistakes that the more relationship-experienced ones have already learned from (and they generally are early 30s when they get snapped up).
The third phase is early to mid-60s. These men are often widowers. Good men who have experienced the loss of a lifetime partner.
In between those life phases, I find that the good ones who don't fit into those moulds are few and far between. Most men who are single during those phases are single for a reason.
My partner falls into the second category. We met online after both he and I came out of long-term relationships, in which we both tried, for many years, to make things work. Had we met earlier, it would not have worked between us, so it was a case of right place, right time. We both consider ourselves incredibly lucky to have met one another when we did.