Midrash sur Le Lévitique 27:3

8) (Vayikra 27:3) "Then your valuation shall be": What is the intent of this? I might think that all who are included in valuations are included in money (vows), and a tumtum (one of undetermined sex) and a hermaphrodite and one less than one month old, since they are not included in valuations are not included in money (vows); it is, therefore, written "shall be."

9) "For the male": and not a tumtum or a hermaphrodite. I might think they are not included in the category of "man," but they are included in the category of "woman"; it is, therefore, written (Vayikra 27:4) "And if she be a female" — either a definite male or a definite female, but not a tumtum or a hermaphrodite. And whence is it derived that the sixtieth year (itself) is regarded as below it? From (Vayikra 27:7) "And if from sixty years and above" — which teaches that the sixtieth year itself is (regarded) as below it.

10) This tells me only of the sixtieth year. Whence do I derive (the same [i.e., "below it"]) for the fifth year and the twentieth year? It follows (inductively). There is liability for the sixtieth year and there is liability for the fifth year and for the twentieth year. Just as the sixtieth year is regarded as "below it," so, the fifth year and the twentieth year.