How do I (25M) let my sisters friend (17F) down without embarrassing her?

Title should basically say everything but basically my sisters friend who is only 17 has a huge crush on me. I always kind of could tell whenever I visited my families house and she was there which was often.
Anyways she asked me last time I saw her if I wanted to basically go out on a date. Before I could even say anything my sister came in the room and she just told me think about it.
Well I don?t need to think about it because she?s 17. But at the same time she?s a nice person and I don?t wanna embarrass her so I guess my question is what?s the nicest way to tell her ?I?m way too old for you? without making her mortified of ever speaking to my sister again? I was thinking I would tell her I promise not to tell my family, but idk, what do you guys think?
TL;DR young sisters underage friend asked me out. How do I tell her I?m too old for her without making her cry?


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