This Insight refers to and discusses the Bi'ur Halcha - Olas Tamid who quotes the Shulchan Aruch (twice) before quoting the SH"A to begin with. This flies in the face of standard Insight convention. (I personally found it confusing as well.)

>>ANSWER: TOSFOS suggests that the three apparently contradictory sources refer to three separate age groups.

(a) A minor may perform Nesi'as Kapayim only together with adult Kohanim (for the sake of Chinuch, to learn how to perform Birkas Kohanim and to become accustomed to the procedure; see Shulchan Aruch OC 128:34). He may not perform Nesi'as Kapayim when he is the only Kohen present.

The BI'UR HALACHAH (DH Aval Im) quotes the OLAS TAMID who says that the intention of the Shulchan Aruch is that a minor may perform Nesi'as Kapayim together with adults even on a regular basis.

(b) A young man who is no longer a minor but who does not have a full beard may perform Nesi'as Kapayim by himself occasionally but not on a regular basis. The reason why he may perform Nesi'as Kapayim occasionally is in order for others to know that he is a Kohen.

However, the Bi'ur Halachah (ibid.) quotes the Olas Tamid who clarifies that the Shulchan Aruch's statement applies only to a place where there are other Kohanim. In such a place, a young, adult Kohen whose beard has not filled out may occasionally, but not regularly, perform Nesi'as Kapayim. In a place where there are no other Kohanim available to perform Nesi'as Kapayim, the young Kohen may do so even on a regular basis (see Bi'ur Halachah, DH u'Mihu).<<