So, a lot of answers will start with it depends.

Buying in: some casinos have a cage in the poker room and others have you buy our chips directly from the table. Ask the person at the stand what their policy is.

Leaving the table: you can leave the table any time you want. Ask the dealer for a color up ( for example get 1 one hundred dollar chip instead of 20 five dollar chips) take it to the cage and cash it out. A lot of people believe it is bad etiquette to "hit and run". For example, winning a big pot then immediately cashing out. I follow that rule if I'm playing at home with my friends, I do not follow that rule in a casino. The point is to win money, I'm not at a casino to be friends.

Age requirements differ. Mostly throughout the United States it's 21 years old due to alcohol being sold. New York, you can play at casinos at 18 where they do not serve alcohol. Canada is 19. If you need to get up and go to the bathroom, then just get up and go. No one will mess with your chips and they'll just deal you out until you come back. If you get up and miss a blind, you will need to post that blind when you want dealt back in