"...This could work, the only problem is that it would be a last ditch effort because you would likely kill yourself as well. Look at it like this if you and your enemy were both 50 years old and you gave all your years to them, then they would instantly age to 100 which in turn would likely kill them from old age. But in turn you would de-age to 0, which would kill you by essentially making it as if you had never been born. Also if you and your enemy were only around 20, then even giving all your Time would not kill them because they would only be around 40...."
In this case, you could probably kill an old man just by touching him and giving him 10 or 15 years.
"Also because the future is fluid and ever-changing you could take in Time and find out that you caught a deadly plague, but you could get rid of that Time and wait a couple of hours and try again and your future might not be the same as it was the first time."
"If I remove time from myself so that i'm say a year younger then break my leg, what would happen when i revert to my previous age? Would it be overwritten, or would some new present be created. " What MountainKing said, your broken leg should be reverted. Though there might be ways to keep the new changes if you wanted."
If I break my leg while im 1 year deaged, and i take a year from my reserve pebble: am I cured? I would always be a few hours deaged, that way the leak would be small and I could avoid any leg breaking really easily.
These two quotations make me think this isnt a case of <the past is written in stone, the future isn't>, and its more of a <What happens when you are your age stays, what happens aged/de-aged may not >, but im not sure I understood it correctly.
"Essentially what it means is that the more Time you pull out of yourself that larger your Difference becomes, and the harder your body will be pulling on the objects around it to try and get back to where it belongs. So the longer you continue to do this the more you will have to constantly fight to keep your body the age you want. Unless you were to wear a suit made of gemstones and be extremely careful to never touch anything, no you could not be immortal."
if I get an insulated stone and put 50 years into it, I would instantly be 50 years younger. For a human body thats a lot so my body would try to absorbe 50 years to get to its right age, right? I already let go of the stone so my body would absorbe 50 years from my clothes and my leather boots, right? If it eats through their timespan they would desintegrate or something like that. If im naked... do I take years from the ground? Like from the earth itself? This makes me think about it a lot like an electricity logic. Is there some kind of time tension for the person that takes years off of himself and uses an insulated gemstone suit?
That takes me to the physical aspect of it:
"So the thing is Time is an almost entirely physical magic. It does not affect minds or the mental side of things at all really."
Surely there are mental consecuences to the physical feeling of de-aging or aging. If you gain 20 years suddenly, you would have a hard time processing the worsening of your joints, your change in metabolism, the weight distribution in your body, as well as things like heart or respiratory complications. I think that would be very interesting as a very distracting set of bodily sensations while you change.
"Also, there is a possibility that she could not even go that far back due to this rule Time can only be pulled out of an object to the point where it last changed form. She might not be able to go back past the childbirth because, in a way, that child is a part of her."
Knowing this, if I was a time user I would take into account and try to learn the permanent changes in my body that would serve as limiters for my de-aging. Maybe I do something to myself just before getting into a time fight, to prevent myself from dying from de-aging. Sadly, it would also prevent me from deaging myself on purpose. Maybe I do that to my rival, with the intention of severing his time skills. Then he would be forced to use some external time and only use his body as a medium in that +1/-1 style.
This applies also to a situation in which I could brake objects or change their state to render them unable to serve as time sources for my rival.
Sorry for the rambling, I really liked this system and enjoyed reading the comments and coming up with strange situations and examples of my own.