As far as the trimurti is concerned, Brahma is almost always depicted as an aged man with white flowing hair and white flowing beard from all 4 heads . Perhaps the white flowing beard depicts his vedic wisdom , as in the proverbial 'wise old man' ?
Shiva on the other hand is mostly depicted as having jatas on head, coiled hair like rishis but no facial hair. However some depictions also shown him sporting pointed mustaches and beard.
Vishnu is perhaps the only god in trimurti without any facial hair at all ! He's shown reclining with a youthful appearance on sheshanag . And so're all his avatars (except Parshuram) shown with a youthful , no-beard appearance. Although sporting long beautiful hair.So much so that one of the names of hair is "Keshava" (kesh : hair)
Also indra, agni , vayu and others when depicted are shown without facial hairs. I am wondering then , why do they not have facial hair ? They seem to be depicted anthropomorphically as adult men . Then how come they don't have facial hairs ? Did they shave (no offence intended)
I have two theories :
1) First, life spans were incredibly big for them. So much so that even at age of 30 or 35, they're actually youngsters without beard actually sprouting
2) Its merely a depiction of the "forever new" and unaging parbrahm. Perhaps this is in conjunction with other attributes like 4 arms (depicting power of parbrahm in 4 directions)