I do not know your situation completely so I will give you my opinion from my situation, with things as they are now (no serious reasons). If I was 42 and without any serious reasons to avoid children, I would not consider the probability of having children with birth defects due to my age to be a serious reason.
The statistics (possibility of birth defects) are there for everyone that reaches that age, God knew that when he made us all. Also, even if the odds are higher than when I was 20, there is always a chance of having a child with birth defects, so when would it become a serious reason, when I turn 30? 35? 40?..
Also, according to a little research I just did online, it looks like the odds of not having a child with birth defects, if do get pregnant at all, is at least 90% (being conservative). So odds strongly in favor of not having a child with birth defects.
Anyways, in genereal, fertility is lower once we reach that age. Furthermore, according to the American Society for Reproductive Medicine, about one-third of women between age 35 and 39 and two-thirds of women over 40 have fertility problems.
The most common chromosomal defect only occurs to 1% of babies born to women that are 40 and only 3.3% of babies born to women who are 45.
My point is that the odds of getting pregnant are low anyways, and the odds of a baby having birth defects are very low. I personally would not consider it just in my case to try to avoid children knowing that I have at least a 90% (again conservative, many places suggested more than 95%) chance of having a child without problems. Even if the odds were different, birth defects would not be something I personally would consider as a reason to avoid children. These children are special gifts, not burdens. Their lives are not less worth living.
Now, even if I were to have a child that does have some sort of “birth defect” this child is a wonderful gift of God. God chose to make that child special in that way and this child would be received as a great blessing for my family, and a sign that God trusts us to take good care of such a wonderful gift. I know you stated that you know children are a gift from God, but I still wanted to add this. I also wanted to add from a Vatican conference on the disabled:
“In the perspective of integral humanism, which faith is able to perceive in greater depth, it is impossible even to entertain the hypothesis that God might have been “mistaken” when he created disabled children. On the contrary, we must say that God loves them personally, and that these children, thus conformed to the suffering Christ, are the object of his special tenderness.”
I know you weren’t saying the contrary, but I just wanted to add that. Also, its always a good idea to seek the advice of a good morally sound Catholic priest. He can help you discern what a serious reason is, though ultimately it is up to the couple.