A minor who has not reached puberty may not lift his hands at all by himself. However if he is doing so together with adult Priests, then he lifts [his hands] in order to learn and practice.
This explanation disagrees with that of Rashi's commentary there, since he explained that they were speaking about the sounding of the shofar, or of an appointment to a position of community leadership, with authority to order lashes or excommunication.
And a person who has reached puberty may lift his hands even by himself. However this is true only on an occasional basis, but not as a permanent practice, until he has a full beard. At that point he may even lift his hands by himself on a permanent basis.
And if one has reached the age where a man's beard would normally be full, even though his is not full, is considered as if his beard is actually full.