When I moved to Illinois at the age of 15, I lost quite a bit of my Texan accent and when we moved back a year later, you could still hear the Chicagoan accent in my O's, especially. It mellowed out, though, after I moved back to Texas a year later. Still, I never quite sounded Texan again after that. I do sometimes put the Texan accent on a bit when I speak to my grandma, I've noticed.
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I think being younger does make it easier to adopt an accent, partly because you are more self-conscious at that age and want to fit in a bit more? Not that I felt I was trying to say things in an Illinois accent, but I dropped a lot of the Texan things to avoid being made fun of, like saying "you guys" instead of "y'all" and that sort of thing, and the rest started to come in naturally. I don't doubt if we had stayed there that I would've picked a lot more of that Illinois accent.