Hesitant about dating him

Hey everyone!
About 3 weeks ago, one of my friends, who I've known for about 2 years, but really got to know him this past summer, told me he feels strongly about me. I never thought of him as more than a friend, but I've been hanging out with him and trying to see where it goes. I have a few issues though that are keeping me from completely dating him openly and proudly. He is about 2 years younger than me (i'm 26, he's turning 24 in a few days). Also, we are both grad students doing our phd...but he started his phd in our lab a lot later than me..in fact I was the one that gave him the position. So it feels wrong dating a junior that I hired..I could never tell anyone in our lab that we're dating, it seems very unprofessional and I feel like it makes me look desperate, dating someone younger, and more junior than me. Also, superficially speaking, he's good looking, but I always pictured myself with a taller guy..and he's only 5'6" (I'm 5'3"). But the stature adds to me thinking of him as much younger than me.

I know he's a very caring, sweet, unselfish guy, and we get along great. But I just don't have that being in love feeling with this guy. Logically I know how amazing he is, but it feels like a comfortable sort of hanging out (no butterflies in my stomach ever)...and I miss having that nervous, I'm in love feeling. What does this mean?? What should I do about this situation? Has anyone been in this situation before that could give me some advice? Thanks in advance

-Hope