reason he gave me for not letting it happen is that he's too old.
Hmmmm, I do not know why we are bashing the guy.
First, he was her "mentor" and he waited until the last day of her ride out to give her his number. This was very professional on his part as he waited until he would no longer mentor her to see if the next step was possible.
Second, when he called, his conversation was very direct. Kudos for that. Why beat around the bush. He asked flat out if you like him and let you know he was interested in you. Major props!
Third, evidently during that conversation or others, he realized the maturity levels were too much for him to handle. If not that, there is something that he does not feel right about the situation. Again, kudos to him as he did not beat around the bush. He told you up front he is no longer interested and to stop contacting him.
How can he make it any clearer? This guy sounds like a straight shooter and I like his path of communication. He doesn't play games or keep you in the dark. He does not need to give reasons or justify anything to you.
If he isn't feeling it, (which he isn't), his only responsibility is to inform you so you can leave him alone and move on.
Also please note your quote above, where you stated he didn't let anything happen cause he was too old. He sounds like a very smart man. Maybe flirted a little realized it wasn't right and backed off. This is totally acceptable behavior. He is not stringing you along and making false promises.
It is you who will not leave him alone.
Personally I think it is your self admitted addictive personality; you are young and new to the field (and the world), and you became very INFATUATED with an older, experienced man. Now the problem is you will not let go and move on.
You also sound very needy and probably have self esteem issues that require constant validation, from the questions you ask and the bit of your conversation to him that you presented to us.
Again this is YOUR issue not his, he owes you nothing as he has stated his position clearly.
Forget about it and move on..you have other more critical issues to fix that affect you more personally than a man ignoring you.