Old people buy blackout curtains because old people are total fucking rebels who resist exploitation by the corrupt capitalist system by refusing to replace their rugs and furnishings every time styles change. To keep the sun from fading all of the perfectly decent stuff they bought in their 40s and early 50s (after the kids moved out), they put up curtains, and only open them occasionally. Eventually, many of them get so habituated to living in semi-darkness behind closed curtains, they quit opening them at all.
Also, paranoia starts to creep in for a lot of old people as their social world shrinks, dementia starts to set in, their bodies experience significant system failure, and they spend more time at home, in front of the TV, watching the world seemingly go to shit. Keeping the drapes closed is a shield against external threats--nobody can see inside and check out all their stuff that they imagine is worth stealing; nobody can observe their daily routine; nobody can see that they are old people, mostly alone and defenseless (even though perennially-drawn curtains pretty much broadcasts to all passerby that you've got old people living alone in a house).