That is really such an individual decision.
I've seen 17 year old kids have an unexpected pregnancy and end up being wonderful parents.
I've seen 30-something professionals have children that they really wanted, but have become so selfish that they end up not being very good parents.
I have a relative who adopted his last child at 47, and is probably one of the best parents I know.
You mention a heart condition. Is that a consideration due to the stress of pregnancy, or a consideration that they may not be around long enough to raise a child? If it's the former, then I would suggest finding a really good Dr. to lay out the risks, and take it from there. Surrogacy and adoption can always be considered. If it's the latter....well, pffffffffft.....any parent can die on any day....there are no guarantees, and the fear of dying prematurely shouldn't be given much consideration.
When Steve and I met, I already had 3 children, all teenagers, and wasn't able to have more. He'd never been married and had not had children. There's a significant age difference between us. It was absolutely a major point of discussion prior to our marriage. He decided that he didn't need to have a child of his own, and if the parenting urge hit him hard later on, we'd look at adoption. As it's turned out (so far), helping to raise 3 teenagers gave him quite a good look at child rearing LOL and he lost any urge to have more. However, now that we're grandparents and very close to all the grandchildren, he's getting to experience a lot of the joys of young kids: teaching them to camp, and handle snakes, and shoot a bow, etc. etc. He feels like he's got the best of both worlds....no real responsibility, but lots of opportunity to influence their lives, and of course he's their hero.
I think everyone should follow their heart. It usually leads in the right direction.