Well if anyone says "Happy birthday America" then the correct age is 406. That number arises from the establishment of the Jamestown settlement, May 14, 1607.

If anyone were to say "Happy birthday USA", then the correct age is 224. The birthday of the USA already happened back on March 4th, since the U.S. Constitution, ratified by September 17, 1787, went into effect on March 4, 1789.

Today, all should say Happy Declaration of Independence from Governmental Tyranny Day. And though Americans celebrate that day on July 4, the Continental Congress unanimously approved by vote for independence on July 2, 1776. 

Jefferson had already written the declaration. A vote to approve his wording happened on July 4, 1776. However, the famous, big John Hancock signature as well as all of the other signatures didn't go down until the date of its signing, August 2, 1776.

So, Happy birthday USA, give or take a few days.