Can you please explain why borderline personality disorder is an "infantile" illness?

Never heard that...: But, if you think about it - it might be that some of the dysfunctional family of origin events that disrupt healthy personality development happen during your very early life. Poor or inconsistent care can destroy trust and leave you with huge needs for others and equally huge mistrust of others. It might mean a bpd person acts immaturely, and some might, but not like an infant. Be well!

Borderline PD: Borderline pd isn't just an "infantile" illness. In fact the developmental period most implicated in this disorder is separation-individuation -- around 18-36 months of age. For none of us is it "a breeze, "and we all struggle to an extent. But bpd gets less support from parents during this time than most do, and there can be earlier development issues also. We all try to get what we need.