From a biological viewpoint, it doesn't seem to make a lot of sense anyway. Some things in the human body do age and become unworkable, we see this ourselves, and at a fundamental level it's possibly a reflection of increasing damage to telemores.
However, generally those aren't hard limits, we don't have a fixed number of days or years before we suffer from a heart attack, or thrombosis.
There are numerical limitations on life, we have ten fingers and toes, two arms, and so forth, but I'm not sure there's anything chronologically speaking that is limited to a certain number, and that number being the same for all of us. Someone with a background in biology feel free to contradict me, but I can't think of a similar circumstance in humans.
On the face of it, there's no reason why he should be hard limited to 12 regenerations, unless it's something utterly unnatural that they did to themselves, with this artificial limitation built in, but the implication seems to have been that this was a natural feature of Time Lords.