I'm 50. I've had quite a few in the last year. The last one is currently downstairs working, as he has turned into a few months stand.
I went through OLD. There are loads of terrible profiles (how difficult is it to take a decent photo to upload in these days of mobile phones?) If they can't be arsed to put a vaguely decent photo and a couple of lines intro, they're filtering themselves out, and it's a busy marketplace for them. A decent photo of their face also cuts out quite a few of the married ones. You will still get ones who talk about sex a couple of sentences into a chat, but that didn't bother me too much for a ONS; it does bother others, so decide for yourself.
It can get tedious scrolling through crap profile after crap profile, so you have to see it as entertainment and share the worst ones with your friends to laugh at too.
I said I wasn't sure what I was looking for, which many of them interpret as wanting a ONS anyway. (But I genuinely wasn't sure.)
I don't think it matters which site you use - you get a similar mix on all, and exactly the same on some. You will get the married ones, the ones who claim to be in open relationships, the ones who want to try out a new kink and stuff, but there are some decent guys out there in between all the rubbish ones.
It's good to meet for a coffee first - enough time to decide if you want more, but not too long if they turn out to be a weird creep or have bad breath or you just don't have any chemistry.
Hotels are a good idea, particularly with children.
I had an STI test before starting and then tested regularly. IME, men will not mention contraception or testing at all, unless you do, and if they're not insisting on condoms with me, then they're probably not insisting on them with anyone rise, either. Some will claim they can't possibly have sex with a condom, which is fine, they have chosen to get no sex. I have a selection of vibrators. They only get to have sex if it's going to enhance my life.
You may well be ghosted by some. This isn't really a problem with ONSs. Or at least, I went a few weeks before realising I hadn't heard from M or R or whoever, so it was mostly mutual. Some came back more than once. Some wanted to come back, but no thanks.
Swinging is an option - single women are usually very welcome, and no one's ever refused to wear a condom, plus no definitely means no. And if you're in a club there are people around if anything goes wrong (never has IME, but I can't speak for others.) It doesn't suit everyone, though, so don't look into it unless you're interested.
ONSs can be great fun or mediocre, but they've mostly been fun, IME. There are risks, but it's up to you to decide where your boundaries are and what level of risk you will take. I probably take more risks than some would, but I don't have children to consider. One reason the current one is lasting is because when I was coming to his house, he had no concerns about me sharing his address with friends, nor with me checking in with them a couple of times during our time together. If someone wouldn't let me share the address of where we're meeting, that would be a concern. Have fun, be safe.