Everything is spirit in some sense. Quantum physics has proven that all matter is trapped energy - hence spirit. Spirit manifestations tend to emanate from the mind - someone's mind, be it the mind of God, a teen, a toddler or any sentient being embodied or disembodied ... perhaps even animals. We've all heard of pet hauntings!
Poltergeists are spirit or energy manifestations that are known for the psychokinetic phenomena that herald their presence. Whether these types of spirits are sentient themselves or simply mirroring the emotions of their "creators" is debatable. I'm inclined to think that some poltergeist's are intelligent entities, whereas others are well ... more like blind forces of nature.
Adolescents (many anyway) are known for being emotionally high strung - but one does not need a troubled teen in the house to experience poltergeist activity. Whereas it's true that poltergeist phenomena seem to thrive in an emotionally charged environment, I'm not convinced that's necessarily what makes them tick.
Psychokinetic ability has been attributed to gifted psychics from various walks and stages of life, and some say it is a "talent" that can be developed in most (if not all) sensitive people. I rather agree, and have dabbled in it a bit myself. Often, the power of concentration with a bit of an emotional investment is all that is needed to effect noticeable change in the environment - whether it be causing a candle flame to flicker or change hue, or slightly altering the ambient temperature in a room - or something much more dramatic. With the right "mind training" many have been able to achieve such effects - and they were certainly not all teenagers.
Uri Geller is a perfect example of such talent. Quantum physics has stated that the mere act of "observing" anything, creates changes in the thing observed - even if only at a sub-atomic level. Considering the popular "butterfly effect," eventually small changes ripple out in space and trigger more far-reaching phenomena. But I digress - that is not the same concept as that of a poltergeist ... or is it? Maybe it is just a matter of degree.
I have certainly experienced poltergeist phenomena - in particular with my daughter, but when she was a toddler and also when she was nineteen years old. I'm not so certain however that she was the cause of the phenomena; not by any means. Once, a florescent light bulb exploded on her when she was about three years old. In the same condo rental, a knife "jumped" off the kitchen counter in plain view, to land several feet away. Smoke alarms went off unaccountably and a toilet would mysteriously flush on its own. At the rental office, I was to find out that particular condo unit was "known" for its poltergeist activity. We were not the only ones to experience it! We were very happy to move out.
Further back, a cottage I stayed at both as a child, and as a newlywed was most definitely the site of sometimes violent poltergeist activity. Drinking glasses loudly exploding on their own (even in a cupboard) and strange noises in the night were part of what I and my family experienced there ... it was too much activity to be chalked up to coincidence.
The house I currently live in also has some evidence of poltergeist visitation. One evening, my daughter and I were startled by the loud sound of four definitely clear and rhythmic "knocks" that seemed to come from near the foot of the couch. No one was there - at least no one that we could see with the naked eye. I don't think my daughter (then nineteen) was the cause of it, in view of all the poltergeist activity I'd experienced in the past. Something, or someone "else" was trying to get our attention! I like to think it was a perhaps distant friend who was displaying some psychokinetic talent.
The following day I Googled the esoteric significance of "four knocks" and came up with some highly synchronistic leads; I even have a hunch as to the source of - and reason behind - the knocking.