Britney Spears' little sister is all grown up!

Jamie Lynn Spears
The former Nickelodeon star turns 20-years-old today -- and marked the occasion by posting "goodbye teens.. HELLO 20's!!!" this morning on her Twitter.

Spears, who lives in Louisiana, says she spent her bday weekend with her adorable nearly 3-year-old daughter Maddie ... and shared a photo of the two playing dress-up in Disney princess dresses.

Happy Birthday!

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