I'm having twins (DC 2&3) soon after IVF. I'm finding it hard enough at 37 so can't imagine what it would be like in your 60s. It's very unfair on the child. I hope to have one, maybe 2 more, but I am going to respect the NHS cut off point of 42 regardless of if I could pay elsewhere. I think it's a sensible cut off point both for my health and children's welfare.
One thing that often doesn't get thought of is that someone in their 60s, as well as being a struggle for themselves (my DM is in her 60s and can only manage a week with a 4 year old), they will often lack an effective support network. Friends and siblings often will have no intention of assisting parenting again in their 60s for anyone other than their own children. And grandparents are usually either dead or too infirm.
It really is crazy. Doesn't that British woman who had twins in her 60s really regret it and advise others against it?