“As Time Goes By” by Abigail Trafford, c. 2009, Basic Books, $25.95, 275 pages, includes index.

The first few notes of the song begin and his eyes meet hers from across the room. Shyly, they smile.

It’s their wedding day, and this is “their song.” She leaves her guests and approaches her new husband. He takes her hand and they dance.

Like any wedding, there are toasts and dancing children, a thrown bouquet and tearful moments, cutting the cake and tender kisses between bride and groom.

But starry-eyed love isn’t just for teenagers and 20-somethings. This is his second marriage. It’s her third. And they’re both in their 80s.

In the new book “As Time Goes By,” author Abigail Trafford examines love, loss and loving again in the second half of life.

Once upon a time, when the average human lifespan was 50 years, people married young and stayed “til death do you part.” Now, at a time when we can live four decades past that old milestone, relationships in the second half of life can give us a second chance at love.

But it’s not easy: Trafford calls later love “The Big Churn.” The kids are gone and it’s just the two of you again.

Everything is old and it’s all new. You want to stay together but need separate lives. Past roles have flip-flopped. It’s like an amusement park ride that both scares and thrills you to death.

Learning to be together anew can be difficult, Trafford says, and old issues can surface with surprising ferocity. Studies show that older couples are more likely than younger couples to “pull the plug” on a marriage that’s unfulfilling. At some point, mortality may play a role: one partner suddenly decides that it’s not an option to live with the status quo in whatever life they have left. But that’s not necessarily the end of love, Trafford writes.

People in their 50s, 60s and beyond are re-creating “normal” with boomerang marriages, throwback romance and serial spouses, triangle marriages, marriage-without-a-spouse, newly renewed unions and fabulously single lives with many love affairs.

Wow.

I read this book with rapt fascination for two reasons: one, because there is an amazing amount of food for thought in this book.

Mixing advice and societal study, Trafford includes interviews with real people, hard research and data and stories that will sound all-too-familiar to anyone who’s had a relationship.

Which brings me to the second reason I enjoyed “As Time Goes By” so much: if you are near or past 50, gay or straight, man or woman, your relationship past is in this book.

Reading it will put a lot of things into perspective – not just for you, but for your friends and children. Surprisingly, because it’s a cautionary look ahead, it’s a good book for 30-somethings, too.

If you’ve ever been in love, if you are struck by the fact that the number of years you have left are fewer than the number of years you’ve lived or if you’re looking anew at an old relationship, you need this book. “As Time Goes By” is simply engaging.