Looks like it's been pinned down to 1760, see here...

There were trousery type things around earlier (Saxons and Jutes) but mostly men and women wore 'hose' - what we'd call stockings or long socks. These were tied around the thigh and the tops covered by the robe or dress being worn. That's why they looked like trousers. The other option was to tie lengths of wool cloth around the legs (like lagging a pipe) to keep them warm.

"Peasants" or agricultural workers would probably be barelegged in summer because a) it was cooler, b) stockings/hose were expensive and easy to snag whilst working and c) little critturs in the field prefer to run to a dark shelter e.g., up trouserlegs so bare legs weren't so attractive to them as a hiding place.

LTS