I love accents! I love how some people can live somewhere for a couple of years and their accent changes completely, and how some people live somewhere for 20+ years and their accents barely change.
My favourite group of accents comes from my friend's family. She is American, he is from the South of England and they live in Yorkshire, so all their kids have really broad Yorkshire accents.
I often chuckle at what people must think when they hear them out and about - it must sound like they've kidnapped the kids!
My MIL and her sister were from Staffordshire and moved down South in the 1960s. They both have really strong accents still.
Me, on the other hand, I have a normally Southern accent, but having spent a few years living in Scotland as a child, whenever I speak to someone Scottish, or spend time there, I can flip my accent. What's really weird is that if I deliberately try to do a Scottish accent I just can't. It just comes out naturally in the odd word or two.
I'd say accents probably change more when you're a child though. Partially because they're still developing, but partly because they'll change it themselves to "fit in" with the other kids.