My brother was diagnosed in his first year at school, didn't help prevent his criminal career later on. Diagnosis, even a self-diagnosis like I did at 42, is just the first step but for kids the right actions and help could indeed prevent so much suffering. Self-medication, drug abuse to calm down the mind, can be prevented quite easily with proper ADHD medication that shows a great rate of success.
Studies from Norway and the US show results fitting this one
https://link.springer.com/article/10.1186/s12888-021-03223-0
https://pmc.ncbi.nlm.nih.gov/articles/PMC2581455/