When did Motley Crue become classic rock?

I think I'm the only one here who knows those are Bowling for Soup lyrics.

When you got old,dude

The term "classic rock" doesn't refer to the music that was released in a specific time period - it refers to the music that's considered to be the most iconic and representative of the time period in which it was released. In colloquial usage, the term often refers to the rock music released during the 60s and 70s, though some 80s stuff may be included, depending on who you ask. But time flies like an arrow. By the time people began using the term "classic rock", twenty years had passed since the release of the earliest classic rock records. Now we're in 2020. 1980 was four decades ago. If people in 1980 could claim that music released in the 1960s ought to be considered classic rock, then it only makes sense that the most important rock songs and bands of the 1980s ought to be able to use that term now that 40 years have passed, (or close to it anyway.) In fact, using the 20 year minimal standard, anything released in the 1990s could technically be considered classic rock today, including a lot of the Seattle grunge acts and their contemporaries. 

As for Ozzy, he does what Sharon tells him to do. She knew getting Ozzy and some of the other Osbournes into the public eye would make them money, so she pushed for it. 

Nirvana is classic rock too.

This my opinion and I have nothing to back it up. I think that if a band's song is still being regularly played 20 years after it was released then it's a classic song. Songs considered classic are proof that it's better to write a song that will become classic than writing a #1 hit that will quickly be forgotten when the next new and temporary sensation makes it to the top of the dog pile. Why? Because the song owners are sill collecting royalties every time their song is played on the radio or some other media.

When rap became mainstream.

Music from the1980s and before is considered classic rock.

About 20 years ago.

I always thought rock music was considered “classic” after 20/25 yrs or something. 

It makes me feel old that’s fo sho.