As a psych major, I will say, there are certain diagnoses we try (and have) to wait until adulthood to make (like the DSM-5 has restrictions) because of how serious of a diagnosis it is and because it legit requires that symptoms and characteristics be THAT longstanding to be diagnosed. While BPD can be diagnosed in childhood and adolescents, it’s a diagnosis that we tend to avoid diagnosing under 18. So, there’s a little why. It’s not necessarily that you don’t fit the criteria, sometimes our hands are just tied unfortunately:(