How old is a 12foot rattle snake with 22 buttons?

Impossible to answer accurately. The segments (or 'buttons') of a rattle-snake's tail only indicate the number of times the snake has shed - not the number of years it's been alive.

And, rattlesnakes do not grow 12 feet long. The largest species, the eastern diamondback, has a maximum potential length of eight feet, which is the record for this snake.