I'm actually pretty interested in it, which is really rare for me to say. Everything that pops up on here seems to be a knockoff of what's most popular (i.e. Twilight and Hunger Games), so a trek to the fountain of youth by a band of thieves trying to earn their freedom is a welcome change of pace.
A few things though. Firstly, I suggest changing the main character's names. I assume you're trying to go for something that's a little science fiction-y, but it ends up looking a lot like any of the other character names that many newbie/young writers go for when trying to pick something that's "cool" and "different". Whatever time or culture they're in, try to find something that looks...legit. That's the best way I can phrase it. Also, you don't have to be showy. The characters' coolness should come from their personality and actions, not names or looks. Oh, and even if it is a couple of hundred years in the future, many of our current names would likely still be in use, if not most. What's common today has been around for generally hundreds of years - sometimes thousands. John, David, Jacob, Sarah, Elizabeth, etc.
I would also include a twist that takes the story to another level, like the King never having intended to give them their freedom and just wanting to use a couple of expendable "riff raffs" to get what he wants. Maybe he even wants them killed as soon as they reach the fountain, in order to better keep the secret.
Finally, give the King an interesting motivation for seeking the fountain, other than the generic "well he's powerful and he just doesn't want to die". Give him some real depth. Whether we're supposed to hate him or maybe even end up feeling for him in some way, he, like every other character, should be a memorable one. No stock characters (likewise, the main duo will have to evolve out of the simple descriptions of "thieves")!
