IIRC, I think it was the late Robert Holmes that helped come up with the 13 limit body. In the 2nd Doctor's era, it seemed to be implied that the Time Lords were immortal. Then again, in the 4th Doctor episode, "The Brain of Morbius", it was also stated by Philip Hinchcliffe, the producer of a lot of the 4th Doc's series, that all the faces seen during the psychic mind wrestling match, were all the Doctor's, implying that he was much older than he liked to make out that he was. Given that in the 7th Doctor's era that it was canon that he was 959 years old when he regenerated, and is now in his 10th body and only 900 years old? Looks like it'll be a good explanation maybe, but I don't mind seeing that artificial limit to regenerations taken off.