Task: Question Answering
Input (English): Context: Marriages improve after children grow up and move out,according to an academic study, which suggests an "empty nest" is not always a bad thing. Popular wisdom has it that parents' relationships may suffer once their young fly the coop, because they feel they have lost their purpose in life.However, a new study by researchers at the University of California, Berkeley, has found that many couples actually feel happier when their children leave home because they are able to enjoy spending time together. In total,123 American mothers born in the 1930s were tracked for 18 years and asked to rate their satisfaction levels shortly after marrying, when they were bringing up babies, once their children reached their teenage years and finally at age 61, when almost all had "empty nests". Although not all said they were happier in general, most claimed their marriages had improved since their children had left home. Researchers believe this is not just because the spouses were spending more time together, but because they were able to enjoy each other's company more. One of the participants in the study,which is published in the journal Psychological Science,said:"Once the kids grow up...there's some of that stress removed...that responsibility removed,so things are a little more relaxed." Psychologist Sara Gorchoff,who carried out the investigation,said: "The takehome message for couples with young children is 'hang in there'." Her coauthor Oliver John added:"Don't wait until your kids leave home to schedule quality time with your partner." However, Dr Dorothy Rowe,from the British Psychological Society,said the effects of living in an "empty nest" will depend on the parents' relationship with their children."If you're just waiting for them to leave home so you can get on with your life,then of course you'll be pleased to see them go," she said, "But if you've built your life around your children you'll be terribly lonely.For some parents,their world falls apart when their children leave." 
 Question: When did many couples feel happier according to the study?
Output (English): at age 61, when almost all had "empty nests".