For a 15yo to travel abroad with a 21 yo

[Elastoslax]

My DS15 and DD21 are planning a trip to France by train this summer.

There shouldn't be any issue wrt to his age as she is an adult, right?

They have eu passports and she has lived abroad independently for a few years so is very capable. I've no worries about their ability to cope but would like to preempt any security bumps. They share a surname but should they also have some sort of permission from parents document?

Cannot find info on this and don't know where to look even.

https://www.gov.uk/permission-take-child-abroad#:~:text=Get%20permission%20from%20someone%20with,about%20taking%20a%20child%20abroad.

There shouldn't be any issue wrt to his age as she is an adult, right?

TBH there might be.

You need to look at the all the relevant country specific rules and also it’s worth looking to see if the travel operator (airline, ferry company etc) have any rules of their own.

There might well be a requirement for a covering, notarized letter.

To add: by way of illustration the following is very generic but it might give some idea of possible rules/requirements…

”In addition to their own valid passport or ID card, all children travelling:

• alone; or
• with adults who are not their legal guardian; or
• with only one parent

may need an extra (official) document signed by their parents, second parent or legal guardian(s) authorising them to travel.”

https://europa.eu/youreurope/citizens/travel/entry-exit/travel-documents-minors/indexen.htm

I think they're fine.
My two have been away with a group of friends with unrelated adults to Majorca. No issues. No letter

DS is going this summer age 17 with friends 18. Checked with both airline (TUI) and insurance and no issues. Insurance says responsible adult and the assumption is that if you trust the person with your child they are responsible. That was Aviva.

It doesn't have to have a letter as per government website but I've not done this so far.

When we've taken my DD's friend on holiday when they were minors I've got the parent to write and sign 'a permission to travel' letter. This included full name of child and their passport number, we were named and would be responsible for the child and included full name of parent, their passport number and contact details. In six or seven trips I've been asked twice for a permission letter at the Eurotunnel and also the border staff has asked the child about the holiday.

I know under 18's to South Africa need a more legal letter from a solicitor even though siblings were travelling together.

I'm also wondering whether your daughter might need something in (the unlikely and hopefully not) case they end up in hospital and someone has to agree for treatment for your son as he's under 16/18?