Shulchan Aruch 128:34 rules that a minor kohen performs nesiat kapayim (lit. 'rasing the palms', i.e. reciting the Priestly Blessing) in order to learn and become accustomed to the practice (as long as adult kohanim are doing so at the same time).
Mishnah Berurah 128:123 notes that this is not at the regular age of chinuch (five or six years old), but rather when the child knows how to 'raise his palms' according to the custom of the kohanim, without giving a specific age or age range.
This seems to leave the age at which a minor kohen begins to perform nesiat kapayim somewhat flexible, depending on the development of the child in question.
However, I remember being told by my father that our family custom was that minor kohanim only start to perform nesiat kapayim during the festival before they turn thirteen.
Are there any (written) sources available describing the practice for minor kohanim to only start performing nesiat kapayim during the festival before they turn thirteen?
Are there any sources describing alternative practices, starting at a different fixed point?
(It would seem to me that starting the festival before the kohen's thirteenth birthday would make the most sense in communities where nesiat kapayim is only performed on festivals. Communities with weekly or daily nesiat kapayim could be expected to have a different starting point (if they have one at all).)