(CNN) - On President Barack Obama's 50th birthday, first lady Michelle Obama poked fun at her husband's age in a fundraising email.
"Every day, I see Barack make choices he knows will affect every American family," she wrote in Thursday's message. "That's no small task for anyone – and more proof that he's earning every last one of those gray hairs."
In the email, with a "gray hairs" subject line, the first lady asked supporters to sign a birthday card for the 44th president.
"Your names and notes will become part of a book that tells the story of this campaign – who's building it, why we're in this thing, and what he means to us," Obama wrote.
After signing the card, supporters are met with a campaign donation page.
The email comes the day after he held two birthday fundraisers in Chicago Wednesday night and on the same day as his birthday party in the Rose Garden at the White House. The events in Chicago served as fundraisers for the Democratic National Committee and the president's re-election effort. The Obama family will pay for Thursday's party, according to a White House official.