Regarding getting the snapshot, I'll quote Dinnerbone:
To get snapshots, open your launcher and press the “New Profile” button. Call it “snapshots” and check the box saying “Enable experimental development snapshots” and save. To switch to the normal version, you can select it in the dropdown at the bottom left corner of the launcher.
Also note, though, that you have to choose 13w38c. There is a new snapshot today, but it has some bugs I'd rather avoid, so we'll stick with 38c until they update again.
Regarding age, I would say go ahead and apply if you feel that you could fit in well with an older group of players. We do have players under 21, I'm just trying to push our average age quite a bit higher as we, the owners, are about 30 ourselves.