The part of the body's endocrine system that regulates the body's perception of time, and through the medium of the chemical messenger chrononine, tells the body how old it is.
Modern Alchemical thought suggests that if the chrononine flow can be halted or even reversed, it would be possible to achieve eternal youth. Alchemists get very excited about this. But then, alchemists get very excited about anything that even hints at eternal life, even drinking their own urine. And in any case the big drawback is that no medical practitioner has ever sucessfully found the temporal gland. Current theory is that it has a sort of non-corporeal existence, like an ethereal appendix.