Voted YES.
I’m in my mid twenties as well and I feel you brother. I’ve been in your shoes 4 years back. Hyundai Verna was my daily driver from when I turned 18 until I was 21. Oh god, that car used to give me horrible lower back pain. I used to always wonder, I’m 18 and I have back pain problems

. My father did get operated once due to a spinal cord issue and I used to think, did I inherit this and am I going to end up at a hospital this young? I also doubted this theory as I never had any problem while driving for 4 hours straight in our Toyota Innova. We drive to our hometown every fortnight to meet my grandparents and this is a four hours drive. We always took the Innova on our highway trips and I never experienced any back pain. I was 6 feet when I was 18, 6’2 now. Few months back, I drove 400km on our Fortuner MT without a break and I had no pain at all. Coming to the Verna, city drives were alright, anything above 40 minutes and I used to suffer from lower back pain. MY father kept telling me sedans are not meant for 6 footers as he had back pain problems when he had the first gen Verna back in 2007 and he sold it to get the Innova and never had any problems (don’t question me on this, I don’t have a scientific explanation, it’s just his experience which turned out to be true in my case).
One fine day, my father took me to his orthopaedic friend (who performed the surgery on him) and the conversation went like this:
Me: I have lower back pain and I can’t bear it.
Dr: You’re too young to have such a pain. Are you lying to skip college? Because I’ll have to give you injections to treat your pain (Dr was a family friend and he knows I hate needles).
Me: I’m serious and I’ll take the injections.
Dr checks my lower back,
Dr: If your mother asks you to get milk or salt or whatever from the grocery store close to your house, how do you go?
Me: Obviously in my car.
Dr: How do you meet friends in your locality?
Me: I drive to my friends house
Dr: Looks at my dad and tells him ‘sell the damn car and buy him a cycle’ and he’ll be alright.
Me:

Dr: Exercise more, don’t sit for long periods, even when you sit, always sit straight and on your butt. These days everyone is leaning and sitting on their lower backs. That’ll lead to horrible consequences in the long run. Have an active lifestyle. Then he begins to rant how stupid millennials are for ten minutes.
Soon after this, we sold the car as I moved to Sydney to do my masters. I used to walk an average of 4-5km everyday there. It wasn’t an exercise, but a necessity. Walking to the train station to go to university, walking to my part-time work place everyday (it was 1km away and there was no bus for such a short trip) or to get groceries. I used to work part-time as a cook at this burger joint, so I used to stand for 8-9 hours during my shift. I had a very active lifestyle and since I moved to Sydney, I never had any lower back pain. Rented out a Camry and drove to the 12 Apostles and never had any problem. I’m not sure how my back pain just vanished. I didn’t take any medication. Maybe it was because of me walking a lot and not driving at all (I didn’t own a car there). Maybe it was because of my very active lifestyle.
Incidentally, a Baleno MT is my daily drive at the moment. 60k kms done and it is still running on the stock clutch. It’s surprisingly soft for its age. It has average under thigh support and zero lumbar support. However, I don’t drive much in Vizag as my workplace is one km away with no traffic and shopping and stuff is also 1-2 kms away. Only time I had slight back pain with this car was when I was driving from Hyderabad to Vizag (700km) and I took only one break for lunch. When I was working in Hyderabad, I used to get slight left knee pain, but this was because of the horrible traffic I used to drive in everyday for two hours. An AT would probably solve this issue.
To conclude, based on my limited experience with sedans, they have lower thigh support when compared with SUV’s. It’s a bit hard to stretch your legs in a sedan too. When adjusting your seat, always try to have maximum under thing support. Unfortunately there are very few mass market cars in India that have lumbar support adjustment. The other day I test drove a civic for a friend and the front seats have the worst under thigh support. I felt like I was sitting on the floor. I’m actually even apprehensive to consider a sedan for my next car. I’m sure there are BHP’ians who are 6 footers and never had a problem with sedans. I guess the experience is different for different people.
In your case, since changing cars has solved your problem, maybe make the change permanent?
Cheers!