Probably multiple causes which are hard to weight for sure at this point. Vaccine shots would be an obvious cause, because they are done at birth (as loony and tinfoil hat as it sounds in today's climate, I wouldn't be surprised if in 50-100 years data about their harm will be collected), a lot of artificial elements in environment (from artificial sweets over-consumed by children to lighting and technology) are likely a contributing factor, just like they are likely a contributing factor to a lot of mental health problems in adults.  And also modern environment allows for more people to be diagnosed and treated differently instead of just living with their mental problems without knowing and speaking about them, which was more prevalent in the past.