A leopard can't change its spots

se sɐɫ pʌɾɯt jʌdɯnqɐdʑi ɡɐndɐ siː sal beɪəʊɹuːt jəʊdjuːŋkeɪkeɪeɪdʒaɪ ɡandɑː

In Korean 세 살 버릇 여든까지 간다 (Se sal beo-reut yeo-deun-kka-ji gan-da)

This Korean idiom, meaning 'A three-year-old's habit goes to eighty,' conveys the idea that early habits are hard to break.