I was 46 when the first of our 4 daughters left, but I sure never felt any depression or sadness. :D A year and a half later we had our first precious grandson and filling up the nest that way is the best way. Each of the girls had a hook on the cellar steps for their coat and as each one left, I magic marked the day they left under their hook.
As far as not being able to keep up with your friends, don't try. Do what YOU love. I'm the only one of our friends who quilts. I never wanted to go back to school, unless it was quilting school. My husband is now retired and we have a blast doing what we want. We do what we want because WE CAN!!!! There is nothing as freeing and wonderful as that!!