What will happen to my 18 year old bipolar nephew who stole my credit card?

Well if he has a good lawyer then that person will try to get mental health treatment as a part of any sentence.

He will probably be prosecuted. I suggest that he hire an attorney.

Bipolar is not a defense to a crime. It can be used as mitigation in scenting if found guilty.

He will probably get a small fine and summary probation. If he used the card he will be ordered to pay restitution but if is a small amount you can probably take case that issue.

The same thing that will happen to a 50 year old without bipolar who stole a credit card. They will be charged with Larceny by Credit Card either over or under $250 depending on how much they spent with it. They may also have additional charges depending on the facts. Many people are diagnosed these days with depression, bi-polar disorder and ADHD. It is the person's responsibility to get treatment with their maladies. Courts do not overlook thefts by people because they have maladies, unless of course you are suggesting that he will plead insanity (that he did not recognize the consequence of his actions). That is a pretty hard defense to proceed with and not very likely to help. He should get a good criminal defense lawyer and let that lawyer guide him.