As far as personas they can be as early as you want them to be. We have several Scythians and earlier.
Until a person proves that they are really interested in the SCA, they aren't going to invest much time in them. I know that seems backwards. The same happened to me when I wanted to start swordfighting. And it's because they will invest a lot of time and energy into a person who says they are interested and then the person just up and flakes out. So, they'd rather have people come and observe and help out at practices (equestrian and swordfighting) and really show genuine interest and dedication.
Then, they'll literally put themselves out just to make sure you get everything you need to live 'The Dream'. They take it very seriously. They are very passionate about it.