My social anxiety sky rocketed at 19 and didn't start to settle down until 23/24. I never knew exactly why, and that's always troubled me a bit. A microbe population getting out of hand sounds possible, as my diet and surroundings changed drastically at 19. Interesting stuff.
People who grew up in a certain SES class (of their parents) also tend to take on poverty-conditions when in college (oh man all I can afford this week are these cup noodles), and not to mention frequent destruction of the microbiome with alcohol. It's probably very disrupting. Maybe has something to do with why those who have a predisposition to having a psychotic break tend to have their first around 20.
>> surroundings changed drastically at 19
Couldn't this be the easier explanation? People at that age tend to move away from home, go to college or start work - all big changes, all highly social and potentially stressful changes. Mental health issues leading on from this is understandable and anecdotally I've saw it in several friends.