Halakhah for Bekhorot 7:28

This commandment is practiced in every place and at all times by male Israelites, but not by females. As we have in our tradition that only a man who is obligated to [potentially] redeem himself is obligated to redeem his son; but not a woman, who is not [in the category of] redemption. The commandment is also not practiced by priests and Levites (Bekhorot 4a) from an a fortiori argument (kal vechomer): If they could exempt Israelites from redemption of the firstborn in the wilderness, it follows that they would exempt themselves. They, may their memory be blessed, also said that even a son of an Israelite who has relations with a priestess or a Levite woman and his son is from her is exempt from redemption (Bekhorot 47a); since the matter is dependent upon the mother, as Scripture made it dependent on exiting the womb. And one who transgressed this and did not redeem his son from when he is fit, that is after thirty days have passed [from his birth]; if he dies before he has redeemed him, he has violated this positive commandment, and woe unto him for he carries his sin upon his soul. And even though there is no specified time for this commandment, as any time after thirty days have elapsed is appropriate, still the wise of heart will grab commandments and perform them at the first available moment, and 'the desire of God will prosper in his hand.' And it seems that the father is always obligated to redeem his son. And even after the son matures, the commandment is incumbent on the father, as the verse states (Exodus 13:17), "And all the first-born of man, your sons you shall redeem" - behold, that the commandment is made incumbent on the father. And so does it seems in Kiddushin.