Everyone I know always tells me now that when I was young, I was such an extrovert and I always talked, but now I’m antisocial. The thing is, for me, at least, I can’t actually pinpoint the time I became an introvert. Most probably the reason why I became one is because I used to talk a lot, but I don’t have a filter and tend to say whatever comes into my head, and most of the time my intentions are misunderstood, or someone thinks I’m rude, so I stopped seeing the point in talking. At any rate, I feel much more myself as an introvert 
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Having said that, I feel like it all depends on the person and the mental state. An extrovert who’s gone through stuff might stay an extrovert, while another extrovert might feel it’s better to become an introvert. 