Travel with granchild-16 y.o. documents needed?

You will need a document signed by both parents of the child, giving authority for travel.

https://www.usa.gov/travel-documents-children

The child needs their own passport and a “Consent Letter for Children Traveling Abroad,” signed by both parents. A form for this can be easily found online.

If I’m reading your message correctly, you intend to travel with the child, and with one parent of the child? In that case I think you (the grandparent) are legally irrelevant. The parent who is traveling abroad with the child needs the other parent’s written permission to take the child there. If it was just you with the grandchild, you would need both parents’ written permission to take the child abroad.

Thank you everyone for the quick responses.

I suspect this is like whether your carryon bag is oversized in that it only matters if someone asks. Obviously, the traveling parent should carry the letter of consent, but don't expect to have to show it, especially with a 16-year-old. I traveled to several European countries with my son, who doesn't have a father, when he was 12, and I don't recall ever being asked.

I did it twice with my son when he was a minor, both times with the necessary letter from his mother. Denmark and Norway did not check it. Canada did. True, you may not need it, but if they ask for it and you don’t have it, it’s not like an oversized bag where you can solve the problem just by checking it or paying a fee. Rather, at best there would be hours of delay while they try and track down the other parent to figure out if they have a parental kidnapping to deal with, or at worst they would not allow the child to enter and send them and the parent straight back to where they came from, in other words deport them and then that would be in their records for the next time any of them tried to come.