Deja vu is sort of like the brain's lag. Think about it like this- when we see something, weirdly enough, the first place in the brain the signals travel to is not the optical cortex, as you'd imagine with the eyes, but rather the amygdala, which is the emotional center. This is where the immediate emotional reactions to things happen, and then it goes to the visual cortex. Of course, we can literally think at a maximum of 250 miles per hour, so we usually notice no difference between our reactions and when we see a coherent image. The only time this difference is markedly noticed is with this bizarre disorder colloquially known as 'blind sight', in which someone's optical cortex is damaged, but the amygdala and the rest of the pathways are fine, so they still react to what they see, but don't actually see it, which is pretty weird. Moments later, these signals go to other areas to provoke a secondary reaction.
So when the brain makes an error, as it often does, it registers the same thing twice. The same signals go to the emotional and optical sensors of the brain with a very short time in-between, though a time nonetheless.
You experience exactly the same reactions twice to two different stimuli which are actually the same, which are processed and stored in exactly the same way, so when the second wave of this same stimuli comes in, it sparks the memories of these same events, just after they've been formed, so you get the sense you're remembering the same thing, which you are, but you're remembering them as the memories are being formed, so you feel like you remember it from before, and the brain's used to being able to recall things properly, so that invokes the feeling of being able to remember the full event, which is still being processed, so you get the sense you know what's coming next, and as the memory fully processes, your brain knows what's happened, so it thinks, 'oh, of course that's what happened', but obviously didn't know before it knew, which is where the feelings of being there or experiencing that thing before, remembering the event and knowing what's coming next even though you can't then experiencing the feeling that what comes next is exactly what happened before.
It is recorded happening in teenagers more as teenage brains are going through a process called neural pruning, in which all unnecessary connections are deconstructed and the brain is open to change and therefore more likely to make spellings, physical, or cognitive errors such as this one. The only other time humans are more prone to this is as young children, but you'd hardly be advanced enough to describe or tell anyone about the feeling at such an age, so obviously, it's recorded happening more in teenagers.
Also, when scientific papers are studied, there are no actual recorded cases of genuine precognition via Deja Vu, and such results have not been producible under any conditions set up in any experiment. Nor has anyone been able to prove they can do that, or been happy to let themselves be tested to see if they really can. Furthermore, while we're talking about neuroscience and psychology, can I try and help but this rumour to rest- we don't use 10% of our brains! We use all of our brain, all the time! I'd like to know where that came from.
I, myself, have experienced Deja Vu most recently answering a question on here, in which I was sure I had already seen the same question, already scrolled down to answer it and started typing.