Women have a biological clock to have kids and if she crosses it the even with the help of the medical science you cannot have kids with her but there are chances that she can get conceived if she is fertile and to confirm whether she could become mother or not, she needs to get some tests done on her. If your only motive to have kids after marriage then she is not the woman for you as there are less chances the she can give you a baby.
Are you in love with her? Do you think your parents would accept her as your wife? Your mother might be of her age so they will never agree for this match. If I were in your place then I would continue the sexual relationship but the marriage is not on the card as that is not feasible for many reasons.
However, the ultimate decision is yours whether to marry her or not but it would be really awkward for you to marry her as that is for sure. After getting married your life may change and down the years you may resent your decision of marrying her so I would say think rethink on your decision keeping your emotion aside. The best thing you can do is, keep enjoying sex life with her as she needs sex partner rather than a husband at this age.