If you moved to a different country every 6 months all your life, what kind of accent would you have?

Your native language, granted that you continue to speak your language whereever you go. Six months is NOT long enough for you to LEARN a language in order to speak it and therefore, get an accent. The wonderful thing would be for you to learn a few helpful phrases from every country you visit. So don't worry.

whatever accent you are used to mimicking. btw it's not "very hard to learn a language after adolescence". Lots of people learn a language after 18 and do fine in them. I myself learned Esperanto after 19. It's probably that english is hard to learn for adults

I imagine they'd have a very unique accent, probably a mixture of the country they last lived in and the one they are in at the moment.

The accent you get is the first language you learn. Sometimes not the case though.

I lived in Britain for a year when I was about 5. two years in America til 7. Korea for 8 years and back in US and this is my 4th year here since I left Korea. when I was about 14, I wasn't even good enough to read a 10-year-old English book. but I could still speak like a native speaker (both English and American). you don't have mixed accent. mixed accent occurs between two languages. maybe some words might have a sound that exist in three languages. anyways, what I'm trying to say is, you don't lose your accent. I've never seen anyone who "loses" their accent. Perhaps if they knew how to speak when they were like, four, and never used that language again. It's really hard to learn a language once adolesence begins. if you're 18 and you lived in a country for six months, you won't learn ****. at least not enough to get an accent lol I've seen a lot of ESL students for more than five years now. usually they didn't speak fluently enough(even though that person can read and write very well) to have an influence on an accent of another language. usually you'll only be fluent(on accents) in languages you learned when you were less than 11~12 years old(it means it starts to get harder to learn a language). P.S.: since I lived most of my early life(a.k.a. critical period) in Korea, I'm fluent in Korean more than English although I lived about the same years in America if you add up.