What is the significance of the five Shekalim with which the 273 firstborn redeemed themselves?
Why don't Leviyim born after this redeem Bechoros of Yisrael born from this time and onwards?
Da'as Zekenim, Hadar Zekenim, Rosh: All future Leviyim are born from Leviyim who exempted Bechoros, but [most] future Bechoros of Yisrael are not from Bechoros, and those born from Bechoros do not know this. 1 Therefore, Hashem decreed that all future Bechoros must be redeemed.
I do not understand this. Had Hashem decreed that those who descend from Bechoros are exempt, fathers would tell their status to their sons, just like they tell them if they are Kohanim or Leviyim! (PF)