I think this is just a decision that you and your lady will have to decide. There are a lot of factors to consider for her and the baby. Even your health plays a huge role in how healthy the baby will be. Being pregnant at 50 would not be easy, I think. I had twins at 40 and it started me into adrenal burnout. The babies were totally healthy and wonderful, but my body took the brunt and I am still recovering 8 yrs later.
I think your lady needs to consider that she will be almost 70 when the child graduates high school. Her sweet, heartfelt emotions might want to give you a baby now, but a few years from now, she may not feel that way. Who can know except the two of you. I would just hope that if she does this, that you will always be faithful to her and make sure you stick around to help raise this child.