My mom has been living in the U.S illegally for 20+ years but wants to go back to Mexico. Would I be able to do something for her to visit the U.S?

I will assume that your mother entered the US illegally or held a nonimmigrant visa with a time certain designation, such as B-2 visitor with a six month expiration date. In such case, your mother would be barred from returning to the US because of her illegal stay. If she wishes at a later stage to return for a visit, she would have to apply for a nonimmigrant visa, be refused, and request that she be allowed to file for a waiver application. It would be up to the consular officer as to whether to allow the application to be submitted and to  recommend it favorably to USCIS for an adjudication. She would be allowed to come into the States if USCIS approved. If you decide that you wish to sponsor her for the green card, you could do it upon reaching the age of 21, but at the end she would require that a waiver be approved based upon extreme hardship to a US citizen or permanent resident spouse or parent. She would not be able to qualify for such a waiver based upon her relationship to you. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.