I was born the same year as Diana so was 19 when she was 19.
I also have a daughter who was 19 in 2023
Yes many of my peers left school at 16 and went into work pretty quickly. Far fewer went to university and on average they may have got married earlier than my dd's peer group. But my dd's generation of 19 year olds were far more worldly wise and savvy than my generation of 19 year olds!
Going to finishing school in Switzerland and living in a flat bought by her father with other Sloane Ranger types was hardly being independent. Her salary from the nursery didn't pay for her life as a single woman. Her life experience was incredibly narrow. Even second hand experience was limited back then. TV was still highly 'sanitised' and documentaries about daily life, relationships, etc. were much, much rarer than now. The only Soap was Coronation Street which was lighthearted. Eastenders didn't start until 1985.
Diana's first hand experience of relationships was her parent's marriage which was appalling!
The only source available to her as teen was probably the Mills and Boone drivel written by her step grandmother 🙄
My friends and I all felt horrified that she was going to be marrying Charles. Not only a man so much older than her but such an apparently dull man. It was so clearly an arranged marriage that we felt incredibly sorry for her.
My peers who got married young, married the people they'd been going out with since school. Boyfriends/girlfriends they'd known for years and were similar ages.
I'm not saying that Diana didn't have agency later on, but the notion that she wasn't young and naive at the time of her engagement/marriage doesn't tally with my experience of that time.