Sazed explains that towards the end of The Final Empire. Sorry if I made some typos.
"If my guess is correct, he would have gained a limitless supply of youth, since he was drawing on most of the power from the metal itself, rather than his own body. All he would have to do was spend the occasional bit of time aged to give himself Feruchemal storages to burn and stay young." "So," Marsh said, "just burning those storages would make him even younger than when he started?" "He would have to place that excess youth inside another Feruchemacal storage, I think," Sazed explained. "You see, Allomancy is quite spectacular—its powers usually come in bursts and flares. The lord ruler wouldn't have wanted all that youth at once, so he'd have it stored inside a piece of metal which he could slowly drain, keeping himself young.
Brandon Sanderson, The Final Empire.