In my opinion, I think maturity should be the first consideration. Age is not the deciding factor in maturity either. *my x is proof of that* Experience is gained by action, no matter what a persons age....but I believe that responsibility comes with maturity, which is most important, especially in a BDSM relationship.
I just read what I wrote and it sounds much like a ramble, so I'll sum it up this way. I don't think age or experience are as important as maturity.