Kate Middleton's birthday: The Duchess of Cambridge turns 30

After attending the gala premiere of War Horse in a full-length lacy evening gown, Kate Middleton steps out of the public gaze to celebrate her milestone 30th birthday in private.

Kate Middleton, the Duchess of Cambridge, talks with guests and dignitaries as she arrives for the UK Premiere of War Horse in London.

LONDON // After attending a gala film premiere in a full-length lacy evening gown, Kate Middleton stepped out of the public gaze  to celebrate her milestone 30th birthday in private.

She turned heads - and dominated Britain's front pages - by attending the London opening of Steven Spielberg's War Horse wearing a design by Alice Temperley. It was another star turn from Ms Middleton, who has appeared ever more comfortable in public since joining the royal family in April.

Palace officials said the Duchess of Cambridge's birthday celebration would be a "low-key and private" affair.

A muted event seems appropriate since the duchess is not just starting the fourth decade of her life but also embarking on her career as a future queen, charged with renovating a monarchy that has become somewhat creaky with age.

"She's done very well, playing a very good supporting role to Prince William and complementing him as part of a team," said Robert Jobson, author of William and Kate: The Love Story. "We've not seen much of her, but when she's in public she has performed with aplomb."

The past year has seen momentous changes for Kate, with her wedding to Prince William and transition from commoner to the world's most talked about princess.

She has won rave reviews for her sense of style and common touch.

But now that the excitement around her wedding has subsided, she is expected to settle down into her royal duties - and for much of this year, that means helping Queen Elizabeth II celebrate her Diamond Jubilee, without upstaging the monarch, and playing a supporting role at the London 2012 Olympic Games.