Have you tried contacting the seller and simply asking to trade back?
How did you find out about this bike and make the deal?
According to the AMA website both Massachusetts and Connecticut require dirt bikes to be registered, which would mean that the bike should have had some sort of state paperwork that would have had the VIN number and model year on it. Did you get any such paperwork?
Do you have any proof that the bike was advertised as being an '03, such as a copy of the ad?
Were your parents involved with making this trade? I assume because you are only 14 and this trade happened across state lines that someone had to drive you.
If you really want to trade back then the best bet would be to take the seller to court. If you managed to make the trade on your own then the easiest approach would be to "disaffirm the contract", where by you basically say that you are a minor and therefore can not legally make a contract such as this. If your parents were in any way involved, however, then the seller can claim that he traded with your parents, not you.
If that is the case then you might still prevail if you can demonstrate that the bike was mis-represented. A bill of sale, copy of the ad, an email exchange, or something similar that proves that the seller was promoting it as an '03 would be essential to this approach, otherwise the seller could simply claim that he had sold it as an '01. The burden of proof will be on you.
The real bitch about taking him to court is that you will need to sue him at his location, and since one of your parents will need to accompany you the required travel and time off may not be worth what you are trying to collect.
As I mentioned above, the bike should have had some registration paperwork with it. If the seller didn't have any paperwork then it is entirely possible that the bike was stolen, which would mean that your problems are only just beginning.
Like 2strokerfun said, this will certainly be a life lesson for you. Don't take anyone's word on what something is, know what you are getting. A little homework ahead of time would have taught you how to decode the VIN and establish what year it was before you completed the trade. You should also have insisted on a Bill Of Sale, that specifically states what you bought and how much you paid for it. If a vehicle is supposed to have registration and/or title paperwork and it doesn't come with it get a "Statement of fact" from the seller explaining why the paperwork isn't available.
rod