I can't answer for the yanks, but the UK started adopting decimal measures for most things in 1971.
However, while the change was made dramatic for currency, it wasn't for measures (ever the pragmatists) and e.g. miles and pints have stayed in common usage, and feet and inches too for quite a lot of us, even people born after 1971 (because our kids are surrounded by people who don't think or speak in metric measures). You might find this website entertaining
https://ukma.org.uk/ - still campaigning for the change, 51 years later
Ultimately imperial measures, so called, are based on body parts, and they are easier to visualize and use than abstract measures based on once-and-ideally-but-no-longer-accurate measurements of the circumference (horizontally) of the earth. (And they were invented by the French, which doesn't help with adoption here of course.)