However on an occasional basis, one who has reached puberty is permitted to go down before the ark, provided that he is not officially appointed by the community, or by a cantor who is hiring him in order to lighten his workload by praying in his place on specified occasions.
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.