Much like “you can’t teach an old dog new tricks,” we sometimes say “a leopard can’t change his spots” to underline that mature people can’t alter who or what they are.
Such a person’s character is too indelible.
The phrase about the leopard’s spots comes from Jeremiah 13:23 in the Bible:
“Can the Ethiopian change his skin or the leopard his spots?”
The expression was first documented in English in 1546.