Roy J. Lester got his first lifeguard job in 1965. He is still at it, patrolling beaches on weekends. But now, at 66, he is determined to keep his thighs covered. Such modesty is a critical part of an age-discrimination case. Lester claims the New York State Office of Parks, Recreation and Historic Preservation wants to get rid of older lifeguards at Jones Beach State Park on Long Island. And to get them out of the lifeguard chairs, it has banned the thigh-covering swimsuit he likes.