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Yep, it's true - Jennifer Aniston is now in her 40s. The Friends alum turns the big 4-0 this Wednesday morning... congrats!

And you know what? She doesn't look any older to us! So whaddya think the actress will be doing for her big b-day? Something with John Mayer, I bet!

In fact, an X17 inside source tells us that the 31-year-old singer wrote a tune for his girlfriend to help celebrate her 40th birthday. Aw, so sweet! Jen's lucky to have a man in her life that can write her songs.

Wonder if it'll be slow and romantic or fast and funny!

Check out our MONSTER gallery of America's favorite sweetheart below...