teach an old dog new tricks
Change longstanding habits or ways, especially in an old person. For example, His grandmother avoids using the microwave oven—you can't teach an old dog new tricks. This expression, alluding to the difficulty of changing one's ways, was first recorded in 1523 in a book of husbandry, where it was used literally. By 1546 a version of it appeared in John Heywood's proverb collection.