"Excited to join Barack in the 50+ club today ... check out my @AARP card!" the first lady told her more than 600,000 Twitter followers in a post that included a photo of her smiling and holding up the red-and-white membership card that bears her name.
President Barack Obama turned 50 in 2011.
Other than the Twitter pic, the first lady spent her big day out of sight, with no public appearances after back-to-back events at the White House earlier this week.
It was probably a good way to spend the birthday, laying low and resting up for a big birthday dance party the president has been planning for Saturday night at the White House.
The White House isn't releasing details but word did leak out that guests have been told to show up prepared to dance.
"Her husband has been spending a lot of time and energy planning it," Valerie Jarrett, a senior White House adviser who is close to the Obamas, told The Associated Press in a telephone interview. "He's so excited and he wants to make sure that the celebration that she began in Hawaii continues through the weekend."
Mrs. Obama got an extra week in Hawaii, her husband's home state, after her family's two-week holiday vacation there. It was an early birthday gift from the president.
The first lady said last month she wasn't certain how she would celebrate her birthday but that whatever she did might involve some dancing, perhaps "a little Dougie" — a hip-hop move.
Birthday wishes poured in over Twitter and Facebook, where women posted messages along with pictures of themselves wearing pearls, necklaces and bracelets, in tribute to Mrs. Obama, who often wears them.
"Happy birthday @FLOTUS! Thanks for helping to make the next generation of children healthier every day!" said a tweet from "Sesame Street." The reference was to her national campaign to reduce U.S. childhood obesity rates.
Jarrett wouldn't comment on reports that Beyonce, who also is close to the Obamas, will sing at the party. That her husband, rapper and entrepreneur Jay Z, performed at the Verizon Center in Washington on Thursday night, added to the speculation.
Milestone birthday celebrations for recent first ladies have ranged from the extravagant to the subdued.
In 1997, then-first lady Hillary Rodham Clinton marked her 50th birthday with a two-day celebration. It kicked off with a party in a tent on the White House South Lawn attended by hundreds of family and friends from all stages of her life. It was followed the next day by a trip home to Park Ridge, Ill. Cheerleaders, marching bands and a children's choir greeted her arrival at O'Hare Airport in Chicago.
Laura Bush turned 50 before she became first lady in 2001. She was still first lady when her 60th birthday rolled around and quietly celebrated with family and friends at what was known as "the Western White House," the Bush family ranch in Crawford, Texas.
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