It's not a matter of if you can get pregnant and carry a healthy pregnancy and baby after 40.
Of course you can. As you've seen, everyone and their mum will tell you all about their success with having a family after 40 or the fact that they know someone who had their first at 49.
It's a matter of whether or not you actually want to have children. It sounds like you don't. It also sounds like your H has hit 50 and had a bit of an existential crisis which may have caused him to question his life, choices and what options he now has. If this is the first real indication that you've had of him wanting children in 15 years-it's hugely unfair to put it on you so late in the game.
It's very easy for men to change their minds on having kids or to suddenly start wanting them. They have a larger fertility window, for one. For two, it isn't their bodies and health that is being sacrificed to pregnancy. For three, typically, their lives can remain relatively unscathed by having a child as the mother takes on the majority of the burden of caring for the children.
That's why it's so imperative for you, as the woman, to be sure that having children is actually what you want. If it's not a 100% yes, it's 100% no.
As far as the "What if you're lonely when you get older and regret not having kids," types of comments-is fear of regret or loneliness ever a good reason to create a life? Isn't it a bit selfish to have a child just to keep you company in your dotage or to provide care for you?
The Jordan Peterson clip (and I don't actually mind him all that much) is off. He speaks from the perspective of a man who wanted a family and wanted children to be part of that family. Therefore, a life without kids for him, would have been quite meaningless and felt empty after 45. That's not really the case for those of us who choose to not have children and don't have any desire to have them. We find our meaning in other places and I have seen countless posts and heard countless parents talk about how lonely they feel so having kids isn't automatically a remedy for loneliness, either.
There is also the longer term ramifications of having children older. Yes, you may be able to successfully carry a healthy child to term. You may also have a good number of years with that child before age truly becomes a factor (depending on how fit/active and healthy you are) but at some point, a lot sooner than many, that child may have to face providing care for their parent/s whilst their friends are off out, partying. Or facing losing their parent/s at a much younger age than they should have to face it.
And yes, of course, even young parents can get seriously ill and need care or pass away, however, there's a greater likelihood of it hitting the child young when the parents are older to start with.
Many will try and convince you that having children over 40 (and 50 in your H's case) is no big deal, but look at the bigger picture and try and scope out the future a bit to see what that could look like. Many people report dealing with increased health concerns after 60. So, if you had a child when your H was 52, that would make the kid not even a teenager and their dad already has issues.
Hopefully you're able to have this conversation with him and be honest about how you feel. It doesn't sound like he's the kind of man to hold the agreement over your head-but if he does, I'd give him a sharp reminder that he's had 15 years to make his apparent desire for children known and he hasn't done so-the agreement is done.