Were contractions used in conversational English in 1800 to 1820? For example: it's, don't, aren't,...
The OED quotes " All want to go to town..—four horses, three mules, and a jack ass—can't carry 'em all." from 1811.
Contractions like "can't" were not often written in those days, so it seems likely that "can't" had been around for quite a few years before that.
Edit: I should have looked further down the page. The OED also quotes " What can I have done with my Papers?..Highty-tighty! I can't have left 'em at home." from 1781.
So certainly, "can't" (at least) was in use by 1800, and probably rather earlier.