Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
54. It is a positive commandment to a Jew to redeem the firstborn son of a Jewish mother
As it states in Bamidbar (Numbers) 18:15, "Redeem, you shall surely redeem, the firstborn of man." This redemption is five Selayim (a coin) or movable property worth five Selayim. The redemption is given to the Cohain and the money is not holy. If the father of the firstborn is a Cohain or Levi or if the mother of the firstborn is the daughter of a Cohain or the daughter of a Levi, there is no obligation to give the redemption. The time for the redemption is 30 days after the child is born as it states in Bamidbar 18:16, "Their redemption is from a month old, he shall be redeemed." If the child has no father, he is required to redeem himself when he grows older (becomes a Bar Mitzvah).
This applies in all places and at all times.