It does sound like his signals were more friendly than they needed to be given the situation. There are several ways to read this situation.
He is romantically interested in you (were there pictures of a wife in his office or a wedding ring on his hand?)
He is excited to have an adult in his class with whom to discuss the class topics and feels he can treat you differently than other students
He is flattered that you love his specialty and is getting an ego boost from talking to you.
- He is a creep.
I Think That He is Flattered
have a very biased leaning towards option #3.
I am a grad student and unfortunately have found my college halls peppered with 40-50 year old men who are brilliant scholars -
which is very impressive in a 30 year old wonder kid but gets a little less exciting by the time they reach their 40's and need to rely on students to give them accolades that they once got from their peers and professors.
I'm not sure this is the case, but I think it is a STRONG possibility. Which is also fed by the fact that you've developed a slight crush on him (I've been guilty of this too!) It is impossible not to be intrigued by someone who is just so SMART and who is passionate about a topic that you are passionate about AND knows even more about it than you do. You want to know more about them, what makes them tick, how they learned so much, and also to hear that you are smart too.
...But I Could Be Wrong
I may be reading the situation wrong.
What do your instincts say? If you think it is more likely that he is romantically interested in you, the person, rather than you, the adoring pupil, and he is NOT married and NOT a creep, check out what your school's rules regarding dating faculty are.
Personally, if you are no longer in his class and will never again be in a situation in which he has the power to give you a grade (or give you a job, or give you a raise, etc.) I think being 36 definitely qualifies you as an adult capable of making romantic choices for yourself.
Good luck. If it is more of the ego-thing your crush will wear off shortly after you are no longer in his class and are exposed to equally brilliant people who may even disagree with him on some finer points of analysis - fallibility is a very powerful tool against hopeless crushes.