Chullin 24 - Five Years for Success in Study
A red heifer was slaughtered by shechitah from the front of the neck, and its ashes were used to remove ritual impurity. The decapitated calf was a needed when a dead body was found in the Land of Israel: the Sages of the nearby town would kill the calf by breaking its neck from the back. Therefore we have a rule: "What is valid for the red heifer is invalid for the decapitated calf, and vice versa."
What is fit for the kohanim is unfit for the Leviim, and vice versa. The kohanim were disqualified by blemishes, but they could serve at any age. In contrast, the Leviim were not disqualified by blemishes, but they had age limitations: they would start studies at 25 and continue on to service at 30. They retired at 50.
The service of the Leviim included guarding the gates, singing in the Temple choir and playing in the Temple orchestra. From them we have a rule that any student who does not see success in the first five years of his studies will never see it. Some say that at this point he should try Kabbalah.
Art: Emil Jacobs - Young Student