Now David was the son of that Ephrathite of Bethlehem Judah, whose name was Jesse; and he had eight sons: and the man was an old man in the days of Saul, stricken [in years] among men.
"David went back and forth from Saul to feed his father's sheep"  (1 Samuel 17:15).  This verse solves much of the difficulty that some have found in understanding this narrative.  The time element is not stated here; but the strong probability is that years passed between that time when David was used for playing the lyre for Saul, and this event, much later, when David fought Goliath.  These were crucial years in David's life, during which he passed from adolescence to vigorous and full-grown manhood.  If, in the meanwhile, David had grown a full beard, that would be reason enough why neither Saul nor Abner recognized him when he went out to fight Goliath.