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.
Thanks for the answer, I live in the UK and it is legal. I guess I was worried about intentions on his side and her getting hurt, however she is very mature and I think that is what the appeal is with being with someone older and the age was unknown when they first started to like each other. They are being quite responsible and are just seeing each other as friends for a while. I agree that the issue I have is whether to support it or discourage it. I have chosen to do neither but inform her instead and allow her to make the decision. I think this is best.
You are assertive when preventing and doing your best supporting her to be cautious and responsible about her decisions. When we are young, even with a good head, the lack of life experiences leaves us vulnerable to many painful experiences, some of them unavoidable but which come at their own time. When we choose to take more responsibilities we need to afford the consequences, the pros and cons of them, so we learn and grow.
Continue to support her just like you are doing right now, and keep an eye open to see how things evolve and the impact such relationship has on her mood, well-being, functioning at home, responsibilities, etc, and keep close communication with her, so you would know what is going on, how to support her better, and how to intervene if necessary.
Thank you for your trust.