The book El "Cantar de Mio Cid" y la épica medieval española by Alan Deyermond, at page 37, mentions
el simbolismo de la barba del Cid
that is, "the symbolism of the Cid's beard", but such a symbol is not explained. I know that the author of the poem mention several times the long beard of the Cid, but I don't know what's the symbolism conveyed in such kind of expressions. It has maybe something to do with maturity? Could anyone explain it?