The answer to your question depends on what concerns you about it.
If your core concern is if it is legal for them to do so, then you would need to review your local state law, since regulating on what is legal or not on this regard changes from state to state, having different age limits, making what could be legal in one state, illegal in others.
Now if your concern is about how sound it is for her to do that and for you to support it, my response is that it depends on how mature, responsible and assertive she happens to be. In most cases, people that young are still learning many things about relationships, intimacy, responsibility around being totally accountable and independent and lack life experiences to support wiser choices, so it is commonly mistakes and pain, what teach us how to better care of ourselves and develop healthy and responsible relationships.
The age difference matters for sure, and as long as it is legal where you live and she happens to meet previous criteria about maturity and responsibility, it could be ok. Just be very aware of the unavoidable challenges the particular scenario presents so for them to take good care of themselves, hold total accountability for their choices and actions and learn and grow from it instead of getting hurt.
Hope it helps.