this will be edited probably but i don't know about America but in England you have to be senior age (18 here) so drinking age in Minnesota (some places 16 for gas refills though) and you have to have a license and/or a briefing and supervision when using a filling station.
18
You probably could fill it, but not to pressures needed for paintball gun operation, CO2 tanks should not be filled with any gas or liquid besides CO2. edit: you could use Argon in a HPA tank but not a CO2 tank
yes you can when you use a compressor only fill up the co2 tank for 2 seconds for every ounce the tank can hold (9 ounce tank=18 seconds) <><><><><> However, the CO2 tank is designed for CO2, not ordinary air, and it is rated for only a specific pressure. If you don't have the proper equipment to do this, it is better for your own safety to leave it to a professional.
If you are not trained on filling Co2 tanks (unlike HPA tanks) you should not attempt this, on risk of breaking the burst disks, or freezing your hand. An HPA tank however, you can simply hook up and fill.
Yes it is perfectly legal for you to get a tank filled. You cannot however purchase a paintball gun till you are age 17. This is just to keep little kids safe.
No paintball gun will work without a CO2 tank or other propellant tank. without Co2 there is nothing to push the ball out of the barrel.
A CO2 tank does not necessarily have to be upright. It depends if you want liquid CO2 or gaseous CO2. When I purchased a CO2 tank for a specific application, namely, CO2 snow cleaning, I had the option of getting a tank that either had a dipstick, that picked up liquid CO2 from the bottom of the tank, or one that had no dipstick so that it collected gas from the top of the tank. I selected a tank without the liquid pickup. So, my upright tank output gas. However, if I wanted liquid CO2, I would put my tank slightly upside down, and get liquid CO2. It is probably not recommended to run a commercial gas tank in a non-upright position. So, depending on what type of tank you have, and what type of output you want, the answer to your question literally is: depends. For farm and garden use, you are probably seeking gaseous output, and probably have a tank with no dipstick; based on these assumptions, yes, you will need to keep your tank upright.
10 2/7 hours
No co2 tanks are sold full.
"H" is not a size for CO2 tanks
One-fifth of the tank
There is no such thing as a "CA" tank.