Because he doesn't consider you his girlfriend and doesn't want you (or anyone else) to think he is your boyfriend.
cuz he doesn't want to be your BF?
Because he's not a boy anymore?
You should ask him yourself. I can't be right but I would think it could be like what everyone is saying: he doesn't want to be your boyfriend or anyone boyfriend in general or he just gave up on love and just wants to be friends with you but with a hint of spice in the relationship, if you know what I mean ;)
His age has nothing to do with him not wanting to be your boyfriend.
Maybe he's in it for something else???
hes married and your his toy