A. The rule in the Torah is to donate half a shekel during Adar towards the communal sacrifices.
What we give these days is a commemorative donation, neither identical with the Biblical half-shekel nor requiring half an Israeli shekel, though the custom is to give coins equivalent to half the local currency, but it is good to give more. The money goes to charity.
The Biblical mitzvah applied to men over the age of 20 (Ex. 30:14), but today anyone can give.