It wasn't aged in barrels at that time. Most of it was homemade and not stored for long before consumption. The source below claims the practice of aging came about sometime post-revolution when whisky was packed in barrels for transit from Appalachia down the Ohio/Mississippi river, and people noticed a difference after it had spent time in transit. Specifically, charred barrels were used to clean out whatever crap was previously stored in there, and that inadvertently created flavor magic. However, you also have aging in barrels in Scotland during the early 19th century, so one development may have influenced the other.

It would have been like moonshine, of variable quality. Unaged whiskey today is called "white whiskey" and you can buy it from various distillers.

Source: The social history of bourbon, Gerald Carson