Sample Text: Megillah 24a
Maimonides Mishneh Torah
Hilkhot Tefillah 8:11
...And one who does not yet have a full beard, even if he is a prominent sage, may not serve as the representative of the congregation in deference to the honour of the congregation. However he may lead the responsive reading of the Shema once he has reached puberty, from the age of thirteen years.
Six factors disqualify a person from lifting the hands: ...and  age...
In what way "age"? A young priest may not lift his hands until his beard is full.
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