I wonder if 13 was just something someone came up with to keep other Time Lords from believing they were truly immortal. If you knew you could die, come back, die, come back an infinite number of times you'd feel there was nothing that could hurt you and you wouldn't give a damn about anyone or anything.
Plus the last couple of regens seemed to be pretty nasty compared to his previous regenerations. 11th's explosive regen - but I do wonder this - maybe there is no limit - but each subsequent regen gets younger and younger until basically you're living your life again as a child. I mean other than the 5th it seemed like the Doctor got younger each time. McGann was younger than McCoy. Eccleston was younger than McGann, Tennant was younger than Eccelston, Smith was younger Tennant... so if my theory continues... then the next Doctor will be younger than Smith - maybe even significantly younger - early 20s even.