Feruchemy revolves around an equilibrium: you can only get out of a metal what you put in to it, and after you're done everything is back in its natural state
Because Feruchemy is a "natural" art, a balance of Preservation and Ruin.
Allomancy does not have an equilibrium: you can get things out of a metal that you didn't put in to it, and at the end the metal is cleanly burned away and the energy is out in the world (extra kinetic energy from steel, for instance).
Because Allomancy is an art of Preservation, the power of creation and sustenance. The metals are burned to fuel the creation of allomantic effects, and IIRC it's even stated that this is a net-positive reaction.
Now, for most Feruchemists, storing age is zero-sum; you can be a bit younger, in exchange for being a bit older.
However, the Lord Ruler was able to perform both Feruchemy and Allomancy at the same time. This created a synergistic effect known in the later books as "Compounding", which pushed him out of the natural Feruchemical equilibrium and towards youth.
In fact, he would have been able to use any feruchemical power in a net-positive manner, by both storing the attribute in a metal and then later allomantically burning the metal while feruchemically releasing the attribute. For instance, this is almost certainly how he managed to heal incredibly quickly: he stored his health in gold and then burned it later.
And as related a side note about a third art, since it's aesthetically related:
Steel Inquisitors, Koloss and Kandra are the results of Hemalurgy, which also has no equilibrium - it's based on the powers of Ruin, so it's a net-negative reaction. Koloss, for instance, have the strength of multiple people but the intelligence of none of them.