On a side note, if you DO have any scholarship money, you will NOT forfeit it when you turn adult. You can still use what monies you have already earned as a legal youth towards college and the like. I'm 25, went adult 3 years ago, and I still have money that doesn't expire till July of 2009. so don't let that affect you.
The decision has to be yours. The leagues work pretty much the same way, albeit a higher cost, and the pots have already been explained. Depending on the league its going to be a little more serious in the adult ranks, but thats something you should chalk up to a growing experience anyways. If you dont have the access to the tournaments, and don't want to bowl with younger kids and not have any competition, I don't see anything wrong with moving up.
I personally stayed till I maxed out, but then there were a lot of youth tournaments and leagues with higher caliber bowlers, and most of my friends were there.
Just make sure its what you wanna do, cause like they said, once you do go up, you cant go back.
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First one to leave 15 ten pins wins!