One in six British workers over the age of 35 are unhappy at work, according to a survey of more than 2,000 U.K. employees by Happiness Works on behalf of human resource firm Robert Half. That’s more than double the number of respondents under 35 who felt similarly.

There are a number of possible reasons for workers’ increasing unhappiness over time. As they age, they either face the growing stress of more senior positions or the disappointment and pressure of not having achieved as much as they had hoped. While salaries tend to increase, so do responsibilities such as childcare, which eats up money and time.

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The survey found that nearly one-third of respondents over 55 do not feel appreciated at work, and 16% don’t have workplace friends. One-third of the survey-takers over 35 found their jobs stressful, whereas only a quarter of those in the 18-35 age bracket felt the same. With fewer years in the workforce, it is possible that young people are still optimistic about their career and job prospects and are not yet burned out.

But it’s not all bad news for the over-35 set. Separate research conducted earlier by Robert Half found that workers in laters stages of their careers feel they possess skills that are aligned with their jobs, exercise greater influence and more freedom at work, and are tapping their strengths—which can contribute to happiness.