It was a tongue-in-cheek response. I don't necessarily think you need 18 months to be sincere. This advisor merely stated you need exactly the opposite of what I would want to see when reviewing a file: recently-begun extracurricular experiences or things having occurred very recently. I want to see a story and if your story seems to start abruptly 12 months before you make this lifelong commitment, I am going to wonder how well you considered it. That is, frankly, what looking at applicants holistically means. Your 4.0 GPA and 45 on the MCAT, or whatever it is now, was meaningless to me without the rest of your application. As premedical students, you see things very differently from how you will see them on the other end.