At 40 I was just starting my downhill slide into pre-menopausal overweight and getting burned out on exercise. At 45 I was 20 pounds overweight and being told to "accept it." Well. I did not. I don't want to be what I was at 35, which was looking like a skeleton, but I think if I had to pick my favorite age, it would be 50. I still feel that I am in better shape now, at just about 62, than I was at 40. Maybe my face looked a little younger at 40 or 45, but not by much. I hate looking at pictures of myself from that decade. I lost the same ten pounds for both of kids' Bar Mitzvahs and the first one's HS grad. I had started cycling about 7 months before his graduation. By the time the second graduated from HS, I was pretty much in the shape I am in now, at 49, almost 50.
And, I have always subscribed to the IDGAF philosophy. It's just easier to do this as you age!