You've hit the golden years, but it feels more like the Twilight Zone. <P> You receive birthday cards from funeral homes. You need three pairs of glasses to get through the morning paper. Your old friends are dropping like flies. On a good day, nothing hurts until your medication wears off. Well, fear not. You're not alone: senior citizens are the fastest-growing segment of the population. You haven't lost your marbles. You're simply having a few Senior Moments. <P> As a public service comes this book -- in large print for your convenience -- to reassure you that you haven't gone senile. You're simply experiencing a second childhood.