To prove you entered the US before your 16th birthday, you can submit:
A passport with an admission stamp showing when you entered the US
Any document from the Immigration and Naturalization Service (INS) or DHS showing your date of entry, such as Form I-862, Notice to Appear
Form I-94, I-94W, or I-95 Arrival-Departure Record
Travel documents, including transportation tickets with your dates of travel to the US
School records from the US schools you have attended, such as report cards or transcripts stating the dates of your studies and the name of the schools
Hospital or medical records from your treatment or hospitalization in the US stating the dates of treatment or hospitalization and the names of the medical facilities
Official records from a US religious entity to confirm your participation in a religious ceremony or rite such as baptism or first communion