"She looks so much older than she is. Lots of wrinkles," one troll wrote.
Meanwhile, another simply asked: "Why is she so wrinkly?"
Acknowledging the Duchess' age, another added: "She needs a little Botox on those deep lines..."
However, royal fans were quick to jump to the defence of the duchess.
Social media became awash with praise for Kate's stylish and festive ensemble.
Commenting on a photo of the Duke and Duchess of Cambridge shared by the Royal Addicted Instagram account, one user wrote: "Kate looks amazing, I love them so much."
Another praised, "Kate always chooses the perfect outfit for every occasion. Always on point!"
"Catherine is always so stunning and elegant!" a third echoed.