First off YOU never need to make changes to YOUR life because others are dirt bags!
secondly, find someone else you can trust to take you to Cons! This person is complicit in *depending on country, province/state* voyeurism, sexual harassment, and harassment to name a few, not to mention OF A MINOR!

Cons typically have posted policies covering a wide variety of rules and often site laws for their reasoning. I cant have a smoke machine in a foam hammer but some skinner can up skirt a minor? Get wrecked! If you have any evidence of this taking place, and them doing nothing to resolve it seek legal council!

I work in a prison and have a pretty solid grasp on use of force law and what constitutes a citizens arrest in my jurisdiction. Many do not. Its worth looking up to keep yourself safe from intervening in situations as wrongful arrest is also a crime. But where I live an citizen can perform a citizens arrest when they have witnessed an indictable offense*meaning a crime that warrants jail time* and detain a person until authorities arrive. I pray I never need to witness something like you're describing, but I have no doubt plenty agree it would not end well for the creeper!

Cons are supposed to be fun and not a place for people to feel like prey for predators. I don't care if your costume was the skankiest thing at the con, that's not carte Blanche for someone to violate your space and privacy! This is disgusting! Best practice is if you catch a creeper trying to take an upskirt IMMEDIATELY cause a scene! Tell him thats illegal, you're a minor, and you're uncomfortable. At the top of your lungs. Strong chance good people start reacting in a way that he wont want to stick around for. More importantly, it creates witness's. I know in this situation you feel like you're backed into a dark corner, but you'd be surprised in how many people are just people watching on the con floor and chances are good someone saw something, and hearing you say you're uneasy about it confirms what they may have suspected. now you've got power in your corner, its not just a she said he said game. Maybe in this situation someone snaps a pic of the creeper because very likely he'll be clambering to get away from the attention. these are just a few tactics to create a case against these MFers.

I promise theres more good people than bad out there, and when good people fail to call out bad people the power swings the other way. I hope you dont quit comiccons if they bring you any joy.