Legal? Yes, in the sense that there is no law against it.
Smart? No, it is incredibly stupid. ANYONE who sees the two of you together and who objects to seeing a 16 year old with a 24 year old, and who has any cause to suspect that you are doing anything with her except perhaps holding her hand as you cross the street, can report you to the authorities. And you do not want to go there, believe me.
And of course, if her parents object, that's the ball game and they can take legal means to keep you away from her. Trust me, they WILL win. There is NOTHING in the law that will force a parent to allow their minor child to see someone they do not wish her to see.
Not to mention the fact that it does not speak well of a 24 year old who is finding that a 16 year old is on his emotional level. And don't bother telling me how mature she is and how this is different; I've heard it all before and it isn't.