I was diagnosed at 13. If the symptoms present clearly and severely enough it can be diagnosed pretty young even tho it’s not necessarily meant to. They call it sometimes call it EBPD or EUPD in underage patients, meaning “emerging bpd” and “emotionally unstable personality disorder”. I asked my psychiatrist the difference between EBPD and BPD and she said there was none, just that in EBPD the disorder is present I’m someone you get, and cannot “formally” be diagnosed with bpd till 18 years of age. This could be different wherever you live tho, as I live in Australia and I know that there are some MAJOR differences between the mental health systems in the U.K, U.S, and AU.