How do you let a younger guy down without it being awkward?

I'm 37.

My friends cousin who is 25, has a crush on me.

He knows how old I am, but he keeps saying he doesn't care.

I don't mind having something casual but he is too young for me.

Should I tell my friend about it?

It's just the age gap that's the problem cause he's not bad looking.

How do you let a younger guy down without it being awkward?
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