Will the Secret Service extend protection for Obama's children after they become independent?

Factual Questions
1

Obama did say that he would stay in Washington, D.C after his term is over to let his younger daughter finish High School without an interruption.

What I wonder however is how things would work out if Sasha and Malia decide to move to Europe or at the least travel alone; With their worldwide publicity as former president’s daughters, is freedom of movement for them in comparison to other average kids impossible?

Do the secret service even ‘vet’ all their friends or classmates? Knowing that they have probably encountered potential violent rapists, murderers around them without any criminal records, how are they kept safe?

In a scenario where they want to live in London, England, what steps would be taken to ensure their security?

2

Sasha and malia get protection until they are 16. Since Obama is almost certainly going to hit the speaking circuit and make millions, I’m sure they could afford to pay for private protection after that if they need it.

3

They’re both over 16, so their Secret Service protection ends at noon on the 20th in theory. But the last two presidents had it extended for a period of time after that.

4

I don’t understand the age 16 cutoff. Very few 16 year olds are living on their own, they’re still in high school. They’re not even full adults yet.

5

Sasha turns 16 on June 10, so she’ll keep Secret Service protection for 6 more months (assuming no extension).