Males could marry at 14 and own property to support their wife, so at 16 Infante Edward (princesses cannot pass along that title, but Daddy Infante Luis can), can rule - but I'm not sure Mary's husband will go along with bypassing her, nor will anyone (parliament) else if Beja presents well, doesn't interfere with English politics and has a level head. I think Mary will still be Queen: look her mother and grandmother were. But, at 16, he would need an strong advisory council (mom & dad?)