I think some people are dramatizing this a little bit and acting like Mom is a common criminal. She helped her daughter open a credit union account at age 14. I bet she was just trying to help her daughter establish credit or giving her a safety net if needed in the future: e.g., access to a credit line while away from home. Remember, not everyone takes these matters as seriously as we do here. It's probable that there was no ill intent or even deception intended. It may have been forgotten about that two names are on the account.
I'm sure they can straighten it out without talk of filing fraud reports, identity theft, etc.
While i agree that she should talk it over with her mom, we do not know anything about the mother; only op can make this determination.
WE are assuming she helped her 'establish' credit but we do not know. So the opinions on here regarding fraud etc are extreme because of the circumstances. OP can tell us what she feels should happen; whether her mom meant well etc.
We just suggested possible avenues since we cannot vouch for the good intentions
If the CC was opened ONLY in your name by your mother then it is identity theft, which is illegal in all 58 states last time I checked, regardless of whether or not you two are relatives or not.
Assuming that you file a police report, you would be entitled to free copies of your CRs as a victim of this. Whether you decide to press charges or not is your choice, personally I would have a heart~to~heart conversation to work it out with her rather than pursuing legal action.
Talk to mom. You may find out she did it for your benefit. And yes, you could very well just be an AU on her account. But for heaven's sake, don't close it out of sheer emotion. You said yourself that you can't get cards on your own and this appears to be the only positive tradeline you have... and a longstanding one, at that. Don't shoot yourself in the foot by closing it just for "the principle of the thing."
If she took it out in your name without good intentions, tell her to stop charging on the account, pay down the balance and give you the online access information for the account so you can monitor it. Then change the address on the account (which I can only assume is hers, since you've never seen a card) to your current address, then a couple of months later--because you don't want to trigger any kind of fraud alert--report the card as lost, get a replacement card sent to YOU, and change the online password. Mom will no longer have access to the account, and you can start using it on your own--RESPONSIBLY.
All good advice here. But first of all, call and make sure it is indeed your mom -- one never knows!