We've had this discussion between ourselves as well. My theory was that while everyone was born with the power of a Zeo, it is not formed until childhood. Events and wishes made by the child can influence what the Zeo becomes later when it shows up. Suki's Zeo is a Healing Zeo. She always wanted to help others and wanted to heal, not hurt. So her Zeo became one that allowed her to help heal others. Carrie's Zeo is an explosive fire-type. If you've ever heard of or seen the MOVIE "Carrie" then you get Pyrokenesis. Hers didn't show up until after a very traumatic event and it was after holding her emotions in for so many years because that's how she was raised. Now it's harder to control and gets more dangerous when she's highly emotional... equalling a highly volatile Zeo.
Emotions and Wishes... As well as Environment can indeed influence Zeos.
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Amyante: Also, i was wondering on how an applying Guardian Tamer actually gets his hands on both a Digimon and a Digivice. Can the Tamer bring his own Digimon, or are they assigned? And how can a civilian get his hands on a Digivice to get his Digimon up to Adult level to even meet the joining requirements? Blackmarket one?
Also, do you have to splice DNA to evolve Digimon, or can they do so using their own gene pool, so to speak? In this case, are they able to evolve from Child to Adult over time, without introducing multiple strands of other Frame DNA types (including ones they already have)?
Amyante: (incidentally, the last bit of text was pasted from the post i made

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Yoshino Suki: Well.... The Digimon can Blast Evolve to Adult Level without a Digivice, so a Digivice isn't necessary for a Digimon to Evolve. Actually.... a Digimon can Blast Evolve to almost every level. They can evolve on their own without a Tamer. It's happened in battles against Non-Tamered Digimon. (It's a term if I want it to be. >_>; )
Suki had Yama before she became a Tamer. Quite a few Tamers were actually friends/"partners" with their Digimon before becoming a Tamer. I do believe you're given a Digivice when you apply in order to pass the tests. You may not be able to keep it all the time until you have passed the Entry Exams and become official, but I'm sure you have access to them for said tests and the like because there's different skills that require use of a Digivice.
"Aging" per se? Not sure, but I know what I just said kind of also answers this.
...............I should post this before Cam thinks I'm really dumb for some reason. XD