Very interesting question and something I've noticed myself. I am Dutch, 27 years old, and started learning English at a very early age (like 9 or 10 yo, pretty common for Dutch children). Up until I went to college (16 yo), I too spoke fluently in English, now I'm tested at "near native" level. And for some reason it kinda became harder for me to keep that fluency. I'm not sure how that developes but maybe being "comfortable" speaking a foreign language, makes you also more "comfortable" to stutter.