It's fascinating to see how much people's experience of the time vary.
Here's my recollections:
I was 3 years younger than Diana, so at school when they married. I went to a state comprehensive, in a pleasant small town. Everyone went to that school - the nearest fee-paying school was 40 miles away, so it was properly comprehensive.
One girl from our year married at 18 - it was regarded as ridiculously young, and generally explained as a means to escape a difficult home life. I'm sure there were other marriages after that, but the next I was aware of, the bride was 21, then a 22 year old bride. Both regarded as marrying young. Between 23-26 it seemed as though everyone was getting married - engaged at graduation, married the following year was a thing. I was married at 25. 27 was seen as old to be single.
I was baffled by all the positive news stories about Charles getting engaged to a teenager. Up till then, everything I had heard about teen engagement / marriage had been negative. I remember reading one article, in which happily married couples with a big age gap were interviewed, and being quite surprised by it.
One of teachers was outraged by the message the Charles / Diana relationship was sending to young women, and told us exactly what she thought.
I think most of my year felt sorry for her in terms of the speed everything happened at - engaged at 19, married five months later at barely 20, then pregnant almost immediately, and William born just before she turned 21. We knew Charles "had" to marry a virgin, which would have ruled out most women closer in age to him, and we knew she "had" to produce an heir ASAP, but it still seemed like a huge burden for a young woman. The idea that she had no sooner lost her virginity than she was coping with morning sickness with no time to just enjoy getting to know her husband and enjoy being married seemed sad to us, especially as she had to be attractive, well-dressed, well-groomed and smiling in public at the same time.