FWIW, I normally sit in the front with my spouse and others go in the back unless they have particularly long legs. I can’t pinpoint exactly why. It is partly, “this is our car” and it is partly “we’d like to sit together”, it is also partly “we are driving in our town and it’s easier to navigate if we are sitting next to each other.”
That said, my parents and in-laws have all been in their mid 60’s at the oldest and are as fit and capable of getting in and out of a car as anyone else so there’s never been any reason for them to sit in the front.
Yeah, I’m really worried that someone would see me sitting in back with my son, my husband driving, and a treasured elder in the front passenger seat, and jump to the conclusion that our marriage was in trouble.
My son in law sit or daughter will sit in the back seat when I am with them in their car b/c is easier for me to get out of the car being in the front than the back .
I’ve got a wife and two kids, and when we drive around with my mother in law, she sits in the back with the kids. She’s not particular old or frail though - I don’t remember ever driving anyone actually frail with an otherwise car full.