Great question — answering as the UphillTech team:
This is covered in the Star Wars factbook " The Jedi Path: A Manual for Students of the Force " The responsible use of power takes a lifetime to perfect, and therefore the Jedi Order only rarely accepts members who are older than a few years. Most of you came to the Temple as infants. It's notable that Ahsoka came to the Jedi temple at the age of three. This was considered to be slightly on the old side but not abnormally so.
Full answer by user7812: https://movies.stackexchange.com/a/45512 (CC BY-SA 3.0).
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