Rebecca Black is All Grow'd Up

Rebecca Black should have NO trouble finding someone to get down with her on Fridays -- or any other day -- once everyone sees how she's uh ... matured.

The singer was out in Hollywood this week looking nothing like the 14-year-old who dropped "Friday" ... the song so freakin' annoying you had to listen to it.

You can see, a lot's changed in 5 years. The now smoking hot 19-year-old is still doing music, mostly released on her YouTube channel.

As for her classic? Still annoying as hell. Now, get ready for the weekend ...