Machatzit HaShekel - Yechaveh Daat 1:86

In this teshuva, Rav Yosef covers a few basic points about the custom of giving machatzit hashekel:

• A person should bear in mind that this is a minhag that is zecher l'machatzit ha-shekel, and not the actual mitzva.
• There is a debate whether a person should begin giving it at age 20, which was the age for the original machatzit ha-shekel in the Torah, or age 13, since at that age a person is responsible for his actions and is in need of a kapara. Rav Yosef sides with Rambam and Ramban who both favor 13.
• The money should be used to support those who learn Torah. Rav Yosef brings several texts supporting this idea, including the Gemara in Brachot 8a that says that after the destruction of the Beit HaMikdash Hashem dwells within the 4 amot of Torah, as well as a beautiful idea from the Maharash Algazi that while a person who brings a korban only brings that type of korban, a person who learns Torah is considered to have given all of the different types of korban.