The term narcissist is somewhat overused, though there are also a lot of them these days. To echo a bit of what others said, thinking average high schoolers are dumb is not a sign of narcissism. Average high schoolers are notoriously foolish. Even if you feel like it’s more than that and it’s a serious problem, that alone does not make a narcissist.
Actual narcissists are unstable. They need the adoration of others to feel good about themselves. They’re prone to fits of rage when anything damages their ego, and they can take just about anything as criticism then decide to fly off the handle.
If that is you, get help from a professional who specializes in it. If anyone reading this knows a person like that, read up and find a way to save yourself.
I don’t have a source handy, but from what I remember: yes, a feral child can learn language later, but never to the same level of fluency. It’s more like learning a second language. Also there is extremely limited data because it mostly comes from horrifically abusive situations.
If I remember right, the most interesting data came from a study that gathered deaf children from areas where they had no sign language. The young children rapidly developed sign language, but the older children (teens) had a hard time keeping up and did not reach the same fluency.