I took some mushrooms with some friends and kind of watched it. I was really happy when Arizona took the lead and then lost intrest so it was alright.
Judy, to answer your questions:
You can play bastardized versions of football. I think most kids in the U.S. start playing football on recess in elementary school (1-5th or 6th grade, 5-11ish) probably at around 7-8 years old. You would not have any of the offensive or defensive lines or running backs, but would be like 6 to 10 person sides with 1 quarterback and the rest receivers or defensive backs (the dudes who guard the receiver). Maybe you would have 2-3 pass rushers who, instead of getting blocked, have to rush on like 5 mississippi or something (a count to about 5 seconds).
By this time kids who are into sports have probably played (in organized leagues) soccer around the ages of 4-5, then most stop, baseball from 5-6 to whenever they quit, and basketball starting at like 6 or 7 to whenever they quit. Flag football starts around 8 and is pretty lame. Then when you are about 12 you can start to join leagues and school teams that tackle. I haven't played tackle football in years, but my last few years of high school we would have lots of pick up tackle games. Again though, there would be no offensive lines, defensive lines or running backs. You would get to hit the shit out of each other though, without pads, which is pretty fun. I remember being really sore after these games and it would probably destroy me as an adult, but it is super fun if you can do it. Lots of people also play like 2 on 2 or 3 on 3 games as adults in the park or something, and maybe in organized 8-11 person teams, mainly flag football (belts with flags that can be ripped of instead of getting tackled) but sometimes tackle.
Basically to sum it up: you can play football without a shit load of organization, but it is kind of dangerous, probably more so as you age, and takes away what could be the three most important areas of the game (both lines and running backs).