The Daily Mail reported Friday the "Beautiful" singer and her husband, Jordan Bratman, both donned the all-white outfit and makeup made famous by actor Malcolm McDowell in the 1971 Stanley Kubrick movie based on the novel by Anthony Burgess.
Witnesses said the couple also spent a good deal of their time Thursday night at a Los Angeles eatery, Osteria Mozza, kissing and hugging one another, the British newspaper said.
Among the non-costumed celebrities joining the apparently happy couple for the birthday celebration were TV stars Paris Hilton and Nicole Richie, the Mail said.
The celebration also helped mark the end of a year in which Aguilera and Bratman become parents through the January birth of their son Max.
Aguilera said recently the newborn has helped the couple grow greatly in terms of love for one another, the Mail reported.
"This little one is a constant reminder of your love for one another," the "Back to Basics" star said.
Notable deaths of 2014 [PHOTOS]
Polly Bergen, Emmy Award-winning actress, dies