Once a catholic woman is too old to become pregnant, does sex between her and her husband become sinful?
Don’t be stupid.
No. It's always a blessing to enjoy sex with one's spouse, even when it will be naturally infertile (a few examples: during the infertile times of the cycle, during pregnancy, after menopause.) That's because one of the two essential meanings of sexual intercourse, is the sharing of sexual love as the expression of the marital bond. As the Catholic Church teaches.