Is men wearing shorts a recent thing?

I live in NZ where we have a very relaxed dress code and guys of all ages wear shorts all the time, but when I was 11 (in 1995) my Grandparents took me to England for a month to see my extended family. My Grandfather was very much a 'proper' gentleman who wore a shirt and tie almost every day until he passed away.
When we got to England, he explained to me that shorts are only worn by young boys and that I would need to wear trousers for the duration of the trip. I accepted this and did so.
However, I have visited a few times since, including this year in July, and saw that tons of adults wear shorts. Is this a cultural thing that has changed, or was it always acceptable? And if it changed, when did it change? My family left England in the 1960s and maybe it changed between then and the 90s and he didn't realize, or is it more recent than that? Thanks for your commentary