Depends on what you mean exactly by "reject." There are mean ways and there are kind ways to communicate to someone that you don't think it would work out (or that you just aren't interested in pursuing anything).
If I were in your shoes, I'd straight up say, "Haha, that's super sweet. But you're too young for me." With a nice, kind smile on your face.
In the alternative: "Nah, you feel too much like jailbait." I'd probably resort to a bit of humor.
That is a very large age difference for a 27-year-old woman. It would potentially be borderline embarrassing, truth be told, if you and he were to become a couple. There's a hint of boy toy there. I don't blame you for perhaps feeling a bit reluctant and uneasy.
You are probably going to need to develop a thick skin and learn to say, no thanks? You are a very attractive woman who will need to say no early in order to avoid greater pain later. I don't know you and have no reason to bullshit you, I'm telling you that based on your very informal but very lovely picture.