The reason for the Fountain of Youth bit is that my MC needed a way to stay live long enough to train a child of prophecy to be the Relic Master. I do agree that the bottling of the water from the fountain does need to have some type of cost and that the potency of it vanishes after only a few years if it is not drunk within that amount of time. My MC is also a person who sometimes forgets important details about things and one of those things is where in his bag he left his bottles of fountain water which is another side effect of the water. Another thing I should mention is that my MC does have a wand that he uses but that is only used as a channel for his magic even though it does have magical properties of its own.

First off thank you for raising these questions. I hadn't really asked myself any of these yet. To answer your first question the lifespan of the castor just shortens. The castor is then able to gain some of their time back through various processes. One of these processes is drinking from the fountain of youth. Though yes the fountain does make you younger in most stories one of my characters is from Avalon, alternate plain of existence in my stories, and the fountain has this adverse effect. Have yet to think of why the magic shortens the castor's lifespan though.

I need some help. I can't figure out how to make a system of how magic works in my books. My characters wield wands but I only want to use those as a focus for directing their magic. Any help would be awesome thanks.