I'm 23 almost 24 and I've learned few things, as u grow older, as u mature fluency increases not sure exactly why, probably cuz you get better at handling your emotions (since as we all know stuttering is somewhat connected to emotions) and u have more experience with stuttering as well, also I've learned increasing vocabular helps with avoiding trigger words or phrases. Also it appears at least from my experience that there are periods when it gets harder as well as periods where it gets easier, and usually periods when it gets harder are usually around that time of year with increased stress like, exams, seasonal depression and shit. But sometimes it happens for no reason, and sometimes as u mentioned with friends and family with whom I usually speak fluently. My advice is relax don't worry, it comes and goes, the more u worry about it the more it gets harder.