After I had the first of my four, I decided that the ideal age to have a kid is when you are two years old so that you will have enough energy to keep up with it! (LOL)
Seriously, I was in my late twenties and while that is not a bad age at all, I can see some value in having children at younger ages and also at slightly older ages. An important consideration, though, is the age of the mother. After age 35, a mother's likelihood of giving birth to a child with various types of problems increases dramatically. But I had my youngest after I was 35, and he did not have any of the difficulties that concern would-be parents.
If you are selecting between financial stability and a solid marriage, I would go for the solid marriage. People who are poor can give a wonderful, loving home to children; people with a shaky marriage will probably have children who are or who become insecure or who have a host of other psychological problems. With a solid marriage, a couple is more likely to find ways to overcome or rise above financial difficulties.
But I would say even more strongly than either of these considerations, that a child needs parents who are mature and who are not self-centered.
So many factors are involved, that it is difficult to give any one answer that applies to everyone.