If you look for 'Matthew Smith' on YouTube, he started stretching as an adult, and now he has extreme flexibility. He trains other people now to get as flexible as he is.
The PT guy I see in person on Monday's only started stretching in his 20's, and he has extreme flexibility now.
I realise I've been stretching for 22 years and still can't do the splits but maybe I just haven't been working on my nervous system properly. The stretch reflex takes place between the spinal chord and the muscle ... you need to condition the spinal chord not to induce the stretch reflex. I'm told the best way of doing this is to work on strengthening movements when a limb is close to its max range.
I'm thinking I'll fork over the US$130 for the side splits flexibility toolkit.