How old did boys have to be to wear long trousers?

It varied over time. Until about the late 50s, boys would wear short trousers until the age of about 12 or 13. That age limit fell during the 60; by the end of the decade nearly all boys changed to long trousers at around 11, sometimes younger. I wore short trousers until I left primary school in 1968. This was pretty common at the time. By the 70s many younger boys changed to long trousers. I wouldn't say you see many boys of any age in short trousers these days, though I can't be sure. Anyway, I hope this helps.