Answer: You are exactly right that unaccompanied youth can access SNAP. There is no age limit and no requirement that youth live with their parents or apply with their parents. A memo from the USDA describes how unaccompanied homeless youth under the age of 18 are eligible for SNAP benefits.
Also, you can view a webinar about SNAP benefits for unaccompanied homeless youth, featuring our own Patricia Julianelle, Ty Jones of the Center on Budget and Policy Priorities, and Courtney Smith of the Detroit Phoenix Center (and SHC Board member).
The length of time a student has a particular address doesn’t affect eligibility. There is no automatic homeless eligibility for SNAP (like there is for school meals). Eligibility is based on income, and “household” size. The definition of household is people who customarily purchase and prepare food together. The SNAP office could look at the length of time a student is in a particular place as part of determining whether the youth is part of the “household” with the other residents of the home. But really, how long a youth is in a particular home is not relevant—the issue is whether they purchase and prepare food together.
Bottom line, this student should qualify for SNAP— but if she is purchasing and preparing food with the boyfriend’s family, they might all be considered one “household”, and that would mean the entire family’s income would count. That might bump them out of eligibility.