We set out to research the causal impact of Real Age feedback, a popular tool on health and lifestyle platforms, on health behaviors. We ran an online experiment where participants were randomly assigned a Real Age that differed in both direction (older or younger) and magnitude (much or slightly) from their passport age, or to a control condition where they received no Real Age feedback. We measured the impact of Real Age feedback on motivation to begin a healthier lifestyle, interest in taking a Real Age test, and percentage click-rate on an optional health link. We found that younger Real Age feedback was associated with higher interest. In addition, participants who received a slightly older Real Age were significantly less motivated to begin a healthier lifestyle compared to not only those who received a much younger or much older Real Age, but also to those in the control condition, suggesting a backfire effect. This effect remained even after accounting for participant health, demographics, and other psychological correlates to motivation. Real Age tests may backfire and demotivate people, and the positive effects they may have on psychological states may not outweigh the negative effects. Though promising, we caution using Real Age tests in their current form as stand-alone interventions to get people motivated.