Why do people make such a big deal about 21st birthdays, if they have already come of age at 18?

Update: There are several answers about 21, being the legal age for drinking alcohol in the US. I live in England where people still make a big fuss about 21st birthdays, although they can buy alcohol, get married without parental consent , and vote when they reach 18. I tried to add this to my question earlier, but YA... show more There are several answers about 21, being the legal age for drinking alcohol in the US. I live in England where people still make a big fuss about 21st birthdays, although they can buy alcohol, get married without parental consent , and vote when they reach 18. I tried to add this to my question earlier, but YA wouldn't let me.
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