To answer the question:
I'm 41, 42 in 3 months. For me it depends on how long the drives are. If I'm playing long open holes, I start tapering about hole #30. If it is mildly hilly and wooded (where you don't want to drive as hard), I can easily do 36 and then another 18+ the next day. It's the next day or 2 after playing that my legs are messed up. For me, it doesn't seem to be quantity as much as it is the pain when the days after. I can be sore the next day after only 18 if it is Blue Valley (10,000+ ft course).
I'm posting because of what I'm experiencing lately, is ITBS - Iliotibial band syndrome. The main symptom was knee pain, but there is significant pain to the outside of my thigh towards my knee. I recently purchased a Pro Tec IT Band. I've only used it for 1 day of 36 holes so far, but it helped quite a bit.
Long drives requires a stronger cross step, pulling it more with your body and legs ,which is where the ITBS started.
I also do have plantar fasciitis but that is more aggrevated with standing on concrete. Playing DG doesn't seem to be too bad.
Stretching is very important. Not just before you play, but every day if you can. I stretch whether I'm playing or not, but especially when my legs are sore.