* Early 20th century: While not yet slang, "awesome" starts appearing in informal contexts with a less formal, more positive connotation than its original meaning. This can be seen in literature, like in a 1919 quote from "The Last Shot": "There was an awesome silence... a silence pregnant with suspense." Here, "awesome" is less about reverence and more about intensity.
* 1960s and 70s: The rise of youth counterculture, with its emphasis on individuality and positive expression, likely accelerated the shift. "Awesome" starts popping up in teen slang and youth publications, increasingly connected to feelings of excitement and enjoyment.
* 1980s: With the growth of popular music and youth culture, "awesome" becomes a mainstay of teenage slang, synonymous with "cool," "rad," and "groovy." It's featured in pop songs, magazines, and everyday conversation.
By the late 1980s and early 1990s, "awesome" has solidified its place in slang, losing much of its original connection to awe and becoming a ubiquitous expression of general positive feelings.
While it's impossible to definitively pinpoint a single year, this period from the 1960s to the early 1990s marks the transition of "awesome" from a formal word to a widely-used slang term.