The wrinkles on St. Jerome's forehead indicate his advanced age.
Indeed, several elements confirm his advanced age: a hollowed and abundantly wrinkled face, a slimmed body with protruding clavicles, a beard and hair in shades of gray and white.
This depiction of the saint in middle age is perfectly in keeping with the iconographic tradition of Saint Jerome, who is often depicted as elderly. The depiction of the saint as an old man underlines his wisdom.St. Jerome is thus compared to a philosopher, giving him a double "identity": that of sage and that of saint.