I’m talking about when a nuclear family of two spouses (husband and wife, wife and wife, husband and husband) and their kids add a grandparent or great-grandparent to the mix. Assume they are fairly spry, and the vehicle is four-door and not super hard to get in and out of.
The grandparent gets dibs on where to sit. Most often this is the front seat because it’s easier to get in and out of. But, wherever the eldest wants to sit, they sit.
I’m not sure why you feel the need to introduce “fairly spry” and “not super hard to get in and out of”.
It’s always easier & more comfortable in the front, and I think the most senior person should be offered the front seat; unless perhaps somebody is pregnant, obese or unusually tall.
The most senior person (in our families, the grandfather) sits up front with the driver.
So no one is of the belief that the spouses who own the car should always stay together up front as long as it’s no great inconvenience for the elder to sit in back?
It depends on who wants to talk to whom (and it’s easier to talk to someone sitting beside you rather than in front of or behind you).
If my husband is driving, I sit in the front seat and my mom and son sit in the back seat.
If my mom is driving, I sit in the front seat and my husband and son sit in the back seat.
If I am driving, my mom sits in the front seat and my husband and son sit in the back seat.
I get carsick very easily, even on very short trips, so I rarely sit in the back seat. I will suck it up if I have to, but my family would rather sit in the back than have me puking.
Elderly always gets front passenger seat. My husband is 6’2, and you can always figure out how to accommodate a tall person in the back-- or just let him drive. I don’t know how pregnancy enters into it. It’s hard to drive when very pregnant, but it’s not hard to sit in the back. I haven’t been pregnant with a two-door; maybe that is hard, but it can’t be as hard as being elderly.
Now, probably someone who is a senior citizen (say, 65-79), but not really ancient would offer the front the front to a pregnant woman, especially in a 2-door situation, but if I were the younger, albeit pregnant, person, I would still defer to the older person, and leave it to that person’s choice.
I remember once being on the bus, and seeing a man who was at the very least, 70, giving his seat to a pregnant woman (at least 8 months), and she looked really glad to get it. I was already standing, in case you are wondering. Now, he could have been younger, and just prematurely grey, and I was pretty young, so I could have over-estimated his age-- the point is, though, she didn’t give him any dirty looks or anything (like everyone on the bus was giving to a guy who was about 30), and then this guy popped up, pretty spry, and said “Take my seat.”
If everyone is able to sit in any seat comfortably, however it shakes out is fine, and words like “belief” and “should” don’t really enter into it.
My mum would be offended if offered the front seat because of her age.
In my family, the front seat usually goes to whoever has the longest legs, which is often me. Even though I’ve explained many times that I can fit in a back seat just fine.
My Mom, aged 72, and my Aunt, aged 85, actually prefer to sit in the back in someone else’s car.
Huh. How come?
My Grandmother didn’t like to ride upfront because seeing the cars whizzing around made her nervous. Also, I got carsick in the back so she always put me upfront. But it was always her call.
Mom gets nervous about Dad’s driving–speeding, sudden stops, excessive lane changes. Sitting in the back when there is someone else to sit upfront makes the driving easier for her to ignore.
Aunt–probably to socialize with the person next to her in the back seat. She’s hard of hearing, and talking over the front seats to the back doesn’t really work for her.
The person with the worst knees gets the front seat but note this is in the UK where cars tend to be less roomy.
For my family the setup has always been “people who have an ‘usual place’ in their usual place, except for Uncle Xavier who always rides in front due to bum knee”. If the person who’d normally ride shotgun is there, they still ride shotgun; take into account that this includes being in charge of the music, AC, nav system and other things which are best done by someone already familiar with the controls. If the usual copilot isn’t present, then the guest is asked for their preference.
Uh… until age 90 grandma sat where she always sat - behind the steering wheel. 
She has only recently begrudgingly given up driving. At age 81 she now sits in the front passenger seat if riding in her car. If in another’s car it is usually the car of one of the residents of her retirement home and seniority rules. That means grandma sits in the rear seat about half the time.
Whoever calls “Shotgun!” first, of course. 
I am.