I'm sorry that you don't believe me acrzybear, but it's the truth. Maybe this news article could convince you. http://sportsbybrooks.com/nude-streakers-are-sex-offenders-in-colorado-20775
You said you were 15, at the time of the incident. That would mean that if you were tried you would have been a juvenile and once you turned 18 you would start with a clean slate and not have to register as a sex offender for the rest of your life for a juvenile prank. That is why juvenile records are separate from adult records, kids make stupid mistakes and unless it is of a grievous nature (usually crimes of violence -murder, rape etc...) you start over as an adult. There is also usually a strict procedure a District (or City) Attorney has to go through to charge a juvenile as an adult. The Prosecuter has to show culpability and certain other elements usually have to be met. Some of these elements include meeting with a psychiatrist and determining whether the juvenile can distinguish between right and wrong, the over all record of the juvenile (has he been in trouble before etc..), what are the possibilities of this action occurring again, and the damage done to society by the juveniles actions. Also the judge would have to ensure that the juvenile offender had a competent Attorney during this whole process, otherwise everything would be overturned on appeal.
As you can see this would be a time consuming, expensive process and is usually reserved for the most serious crimes that spark public outrage. Now you might be one of the folks that fell through the cracks (it does happen), but personally if all I did was "moon" someone as a juvenile and I was put on a sex offender list I would do everything I could to get off it. This is why there are juvenile records, kids make mistakes and unless it is serious it shouldn't be held over their head for the rest of their life.
Whatever your history is, good luck with your future endeavors.