In Elizabethan times the age of consent for a man to marry was 21 and this is largely why the birthday still holds so much significance now days as although this law has changed, the 21st birthday is still seen as the age when the boy truly becomes a man.
Nowadays the traditions have changed and the 21st birthday is seen as a time for a big party and celebrations care a little more raucous.
In America whilst many teens experiment with alcohol in their high schools years the legal age for buying and drinking alcohol is 21 and so it is tradition to enjoy your first 'legal' drink on your 21st birthday. This tradition is often hazardous to one's health as the birthday party goes into the late hours of the night and a large quantity of alcohol is consumed. In fact tradition has it that as it is the 21st birthday, the person whose birthday it is should consume 21 drinks, quite often shots of straight liquor.
Whilst in the UK it is customary for the person whose birthday it is to give a little speech to thank everyone for celebrating their birthday with them, in Australia it is customary for the person whose birthday it is and all of his family and loved ones to each give a speech after cutting the cake.
Parents are expected to talk about how proud they are that their child is now an adult, partners give a speech about how much they love the person and look at their lives going forward and friends are given the right to tell embarrassing stories about the birthday boy /girl.
One tradition from the past that lives on today in many countries around the world is the giving of a 21st birthday key. The tradition originated many years ago and the gesture of giving the key was that at the age of 21, you were considered to be old enough to own a key to the house and a senior member of the household.
Whilst modern times have made the meaning behind the gesture a little obsolete given most people own keys to their houses a lot younger than 21, the giving of the key on the 21st birthday remains and these are now sold in all different sizes and finishes by gift shops all over the world.