Agreed with that timeline, fileboy. I'm 42 (training since I was 15), and didn't really have too many problems until I was about 39 or so. It does add up. Mixing the arts is fine, but just try to be careful with the intensity. Don't over do it!
In Poland you have to pass a sports trainer's course in whichever disciplines you are teaching. It includes knwledge of anatomy etc. Otherwise, you can't teach.