Thousands of baby boomers are finding themselves having to choose between expensive monthly premiums that rise with age, and cheaper plans with skimpier coverage and high out-of-pocket costs for doctors and prescriptions. Consumer advocates had hoped that regulators overseeing Massachusetts' health insurance overhaul would start to address this issue, by making it easier for self-employed people and retirees who are 50 to 64 to be exempted from a stiff tax penalty if they can't afford insurance. Connector Authority board members recently postponed tackling the issue, however, saying they needed to further study it.