My teenage sons have different last name than mine and 50% of the time I was asked for the letter by U.S. Immigrations.

I was denied boarding to my Xmas cruise in San Juan (U.S.) because I had forgotten the letter and had to make frantic calls to have my adult son at home fax over the notorized letter. My son is 13 year old

I was also asked for it a couple of times when I drove across the border.

Definitely have one, you never know.