Enrollment in private Medicare plans offered by insurers led by UnitedHealth Group Inc. (UNH) is expected to surge 11 percent in 2013, the U.S. government said, while premiums may increase an average of 4.7 percent. About a quarter of Medicare’s 49 million elderly and disabled beneficiaries are enrolled in Advantage plans, in which insurers are paid a fixed monthly fee to provide services. Some of the plans offer benefits not available in the traditional Medicare program, such as discounts for gym memberships and eyeglasses.