One thing that can work against you is the time--as more time passes, the ability to produce paperwork decreases. However, I think you can show this easily enough. As far as the law goes, under normal conditions, a minor--under 18--has no legal ability to enter into a contract. So, what records can you find from the lawsuit? Here's what I would seek to show at the hearing....
1--if you can show the date that the lease was signed, you can show your birth certificate to the court. That alone could be enough, as it would prove that you could not possibly have been financially obligated for this debt. Look for proof of this in their own claims---if they are garnishing wages, there had to be a lawsuit. Check to see if you can find the records of that case. I'd be willing to bet that somewhere in their complaint or in supporting evidence, you would likely find either their statement on when the lease was commenced, or maybe even a copy of the lease paperwork itself. If so, you have strong evidence that they stole money from you.
2--The date of service is another key. It stands to reason--simple common sense--that the date your mom got served with the summons HAD to occur some time AFTER the debt was actually incurred. So, I would look to see when the complaint was actually filed, and when they served your mom the summons. Then, compare that to your date of birth and show the court that you were not old enough to incur this obligation. Once you can show that you were a minor when all this happened, you can request that the court order return of all money they took from you. Explain to the court that you're being very seriously hurt financially by their misconduct.
3--I would also maybe consider filing a lawsuit of my own against them at this point. You can show money damages because of their fraud. They are NOT allowed to garnish your wages for someone else's lawsuit as long as they cannot show that you were legally connected to the debt itself. But the most important thing for you now is to look through their lawsuit if possible and get the info that shows you were not even legally able to be on the lease---or, if it's in the case, get the basic info. Example---if they did not sue you, but only sued your mother, then they cannot garnish your wages anyways because they have no judgment against you. It might be enough to simply show that the judgment they are relying on to take your wages does not even have your name on it anywhere.
