Midrash su Levitico 21:17

1) (Vayikra 21:17) ("Speak to Aaron, saying: "A man of your seed, to their generations, who will have in him a blemish, shall not come near to offer up the bread of his G d": ("A man of your seed"): R. Elazar b. R. Yossi said: A child is unfit even if he is unblemished. When is his service kasher? Once he grows two (pubic) hairs. But his fellow Cohanim do not initiate him in his service until he is twenty years old.

2) "who will have in him a blemish": This tells me (that for his service to be kasher) he is not to have a blemish after the pronouncement (of the law), as in the instance of zivah and negaim (plague-spots). Whence is it derived (that his service is not kasher) if he has the blemish before the pronouncement? From "in him a blemish." This tells me only of his having been born unblemished and having become blemished. Whence do I derive the same for one who was born blemished from his mother's womb? From "in him a blemish."

3) "he shall not come near to offer up the bread of his G d": This tells me only of the temidim (the daily burnt-offerings), which are called "bread," (viz. Bamidbar 28:2) "My offering, My bread for My fires." Whence do I derive the same for the other offerings? From the repetition of "bread" (here). Whence is the same derived for the (sprinkling of) the blood? From "to offer up" (here) and (Bamidbar 9:9) "And the sons of Aaron offered up the blood to him."