Well, about admin, you need to be pretty respected around the community and probably of had a lot of experience before, as well as be an older member so you have a general idea of the forums and etc. It is a very rare chance that you, (this goes with pretty much everybody), will become admin, as you have a lot more responsibility rather than being a local mod etc.
By mod, I'm assuming you are talking about Super Moderators and stuff like that. Those guys are chosen more easily rather than Admins who are very carefully chosen. Anyone has a chance at becoming a moderator, as long as you know the position you are applying for and you have some experience around the forums. For example, someone that is more respected around the community and has a bunch of posts and an acceptable join date (probably '10 or earlier) has a greater chance than a new member who applied, but it also really depends on the application and whether it was very good or not. These positions are not to be taken lightly, because you are gaining power within a large community of people, and I'm pretty sure that everyone would like a stable section of staff members who know what they are doing, rather than some new guys trying out a position.
I hope I answered your question clearly enough.
~Faint