They do link to a few scholarly studies that agree with their conclusions as a company, so I don't feel like this is totally reliant on the app users. Here is one of them that I was able to download and read: https://www.academia.edu/3633328/The_holistic_phase_model_of...
As an anecdote, when talking with my friends, we typically agree on that the time in secondary school - around the years 13-16 was the worst time in the life, defined by angst and worries, and those that went to university typically says that was the most fun times, because of all the new friends and all the activities.
But maybe no one asks the kids how they really are doing.
But maybe no one asks the kids how they really are doing.