Traditionally in England, people were considered to come of age on their 21st birthday. In modern Western countries that used to be English colonies, you are legally [by law] considered to be an adult on your 18th birthday (in some states of America it’s still 21), however, many people still hold a special party for their 21st birthday.
In Australia & New Zealand, 21st birthday parties are big events. All of your friends and family are invited to the party. So many people come to the party, it usually has to be held at a special party hall or in a marquee [a big tent] in the back yard of your parents’ house.
At the party, your father gives a speech in which he formally recognises you as an adult. As a part of this speech, he usually “gives you the keys to the house.” This is showing that you are now an adult, free to choose when you’ll leave and come back home. Key’s are an important symbol at 21st birthday parties.
Another common tradition at Australian & New Zealand 21st birthday parties is to scull a yard glass. “To scull” means to drink without stopping. A yard glass is a special beer glass that is about 1 meter long. It holds about 1 ½ to 2 litres of beer.