Why is Brahma depicted as old in pictures?
Brahma is often referred to as "pitamaha" (forefather or grandfather) in scriptures, so I think some of the pictures that portray him in human form with white hair and beard is to just show how he is older than some of the other gods.
From Valmiki Ramayana, here are a few verses that refer to Brahma as pitamaha:
इति एवम् उक्त्वा देवेशः सर्वलोक पितामहः |
यथा आगतम् तथा अगच्छत् देव लोकम् महायशाः || १-४४-१६
16. sarva loka pitaa mahaH = for all, worlds, forefather; mahaayashaaH deva iishaH = great glorious, gods, lord of - Brahma; iti evam uktvaa = thus, this way, on saying; yathaa aagatam tathaa = as came in, like that; deva lokam agacChat = to empyrean, world, went away.
On saying in this way, the forefather of all the worlds, and the great glorious lord of all gods, namely Brahma, went away to his empyrean world as he has came. [Bala Kanda, Sarga 44, Verse 16]
ततह् स्वायम्भुवैर्मन्रैर्ब्रह्मस्त्रमभिमन्त्रितम् |
हनुमांश्चिन्तयामास वरदानम् पितामहात् || ५-४८-४०
40. brahmaastram = (knowing it) to be missile presided over by Brahma the creator; abhimantram = and consecrated; mantraiH = by spells; svaayambhuvaiH = sacred to Brahma the creator; hanuman = Hanuma; tataH = then; chintayaamaasa = recollected; varadaanam = a boon got; pitaamahaat = by Lord Brahama; the grand-father of the entire creation.
Knowing it to be a missile presided over by Brahma the creator and consecrated by spells sacred to Brahma, the creator, Hanuma then recollected a boon got by Lord Brahma, the grand father of the entire creation. [Sundara Kanda, Sarga 48, Verse 40]
This is no different from how Bhishma is depicted in paintings.
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