You Can't Teach an Old Dog New Tricks
(elderly people can't change their behavior or learn anything new)
To many of his friends, Mr. Jones seemed quite eccentric. Although he was getting on in years, he had a long-standing habit of going out alone to walk in a shoddy part o f town late at night. His friends warned him that this was a dangerous practice, and they advised him to do his walking during the day. However, he made no effort to change his routine. It was evident that you can't teach an old dog new tricks. Even his closest friends had to accept the fact that it's almost impossible for an elderly person to adopt a new routine.
get on in years = get older
shoddy = 見かけだおしの、安っぽい