“I have a confession to make,” Bill Thomas announced several months ago at a conference on aging in Oregon. “I am an old man.”“No, you’re not!” an audience member called out. It was meant, no doubt, as a compliment: Despite his gray-streaked beard and crow’s feet, the 56-year-old geriatrician-cum-thespian crackles with high-octane energy. And isn’t that what we all want to hear as we age? That we don’t look old? That we seem younger than we are?It’s not what Thomas wants to hear. After more than 20 years of trying to make life better for old people, he believes the correct message is the opposite: That we are lucky if we get to grow old. That there is a “third” phase of life beyond adulthood that can be as rich as either of the phases that came before.