The firstborn were already sanctified in Egypt (See Sh'mos 13:2). Why were they redeemed only now?
Ramban: Because until now, bearing in mind that the Kohanim had not yet been appointed, there was nobody to whom to give the redemption.
Ramban (based on Zevachim 112b): They were sanctified to offer Korbanos 1 . Now [due to Chet ha'Egel], Hashem chose Levi in place of them., and they were their redemption.
Which firstborn is the Pasuk referring to? When were they born?
Which firstborn animals Bechoros did the animals of the Levi'im except from the Bechorah?
Rashi: They exempted the Pitrei Chamor (if not, Yisrael would have been obligated to give a Seh to a Kohen to redeem each firstborn donkey). 1
Rashi: One lamb of a ben Levi was sufficient to redeem any amount of Pitrei Chamorim of a Yisrael. Proof of this lies in the fact that the Torah does not count the leftover lambs of Yisrael.
QUESTIONS ON RASHI
Rashi writes that the Leviyim's animals exempted only Pitrei Chamorim of Yisrael. In Bechoros 4a, Abaye learned from a Kal v'Chomer from the Midbar that Seiyin of Leviyim are exempt. Rava said that if so, even their Tahor Behemos (i.e. also cattle) should be exempt! Also Rashi explained so there!
Moshav Zekenim citing R. Avigdor: The Kal v'Chomer was, if even a Pashut (non-firstborn) Seh of a Levi exempted a Peter Chamor, all the more so it should exempt its firstborn from Kedushas Bechor. Rava said that if so, even a Bechor Seh of a Levi should be exempt. Do not say that they exempted Yisrael's animals, for there is no Pidyon for a Bechor of a Tahor Behemah! There is Pidyon only for human Bechoros and of donkeys.