That assumes that old people live at home / have a regular mail box.
What if they live in a nursing home or similar? Although 4x6 (10x15cm) is postcard format, so usual mailboxes should fit quite a number of those just fine.
The photos come in a pretty much regular sized envelope that fits in small mailboxes. I do like the idea of letting people pause a delay a delivery for whatever reason they want, or maybe even redirect it at the last minute if needed. Appreciate the feedback and ideas.