At the age you stop training it appropriately. Many people never train it appropriately, but still keep it. I've seen a lot of folks in the martial arts and dance world in their 70s, 80s, and 90s that can still do full splits. I'm much, much more flexible now (getting close to full splits) than I was when I was in my teens or twenties. It's all about having the right methodology and following it. There are lots of folks who didn't achieve a split until they were past 40.