Why did DSM-5 and subsequently ICD-10 allow personality disorders to be diagnosed in children
An article on the NCBI website states


  Many clinicians are reluctant to diagnose personality disorders (PDs) during youth, viewing paediatric personality deviations instead as reflective of given developmental stages. This is so despite evidence that certain youth are indeed at risk for the eventual development of PDs as adults. Unfortunately, late identification of these disorders prevents timely treatment and potentially increases morbidity.


There are also a fair few references to articles backing the idea up