I am a 65 year old female with a diastasis recti. At my age, can it be fixed?

Tummy tuck is not painful. In a healthy person tummy tuck and diastasis repair can be done at any age.  However, your abdominal bulge is likely to be from your weight. At your weight operation will not succeed.  If you can get your weight down to 150 lbs you will have a successful operation.  

If you are a healthy, non smoker, your age itself is not a contraindication to surgery. However, your BMI is 40 and that is a significant contraindication to surgery and greatly increases your risk of recurrence of the diastasis and complications after surgery. 

Your BMI needs to be closer to 30, if not at 30, before your proceed with Diastasis repair. At your current BMI, you’re intraabdominal contents (intestines, organs, and the surrounding fat) will push against your repair and that pressure can tear your repair apart. Even if you had a surgical repair, your abdomen may still appear round if your BMI is high because your intra-abdominal contents haven’t gotten smaller.

Sounds like you have a great doctor who gave you good advice.