As noted in the comments, the film was released in 2010, but the production extended over some 15 years, so her exact age in that scene is not firmly established. Somewhere from 53 to 68.
Sally’s first nude scene was (to my knowledge) in 1969’s Futz (age 28 or so), and her latest was in 2019’s Cuck, when she was nearly 80. Although we know her mainly as a full-breasted blonde, she had dark hair and a thick matching bush when she exposed her modest B cups in Futz. As you can see above, her breasts had grown substantially in the following thirty or forty years. Gravity appears to have had little or no impact on them. It’s another Festivus miracle.