Ok so for two and a half years now I have had this friend, a guy, (I'm a girl) who's just under 5 years older me. At first I fancied him - he was my first crush ever - but he had a girlfriend and eventually I got over him and started to view him as a friend, a brother. 

More recently, since... I don't know maybe April? Late march? Things started to go a bit pear shaped. He kept ignoring my texts and emails and never had time to talk to me and we kept arguing whenever we did talk. After a while of this, we talked about it face to face and put everything behind us. We also figured out the following: 

I valued myself on how he viewed me, for example if he took the time to talk to me it would make me feel happy and worth something but if he ignored my texts or emails I would feel undervalued and annoyed or upset. 

He said I respected him too much, and therefore he was going to stop talking to me for a bit. It really upset me, and I dwell on it a lot. That's not the point though. What is the point is that after dwelling, I realised that, as well as what we had figured out together, that:

If something was going on in his life and he told me about it I felt good because that meant he respected me and trusted me enough to tell me, and he didn't think I was too young to understand. If he didn't tell me I felt that he disregarded me as being too young to understand or not a good enough friend to trust. 

Similarly if he was upset by anything and I couldn't help him or he wouldn't talk to me about it I would get distressed. 

Over the last year and a half I have really valued his advice more than any one else's because he taught me so much of what I know about the world today and he helped me through some really tough times for about a year. Didn't know at the time why I wanted HIS advice so much, I thought it was just because I'd known him so long and I was used to going to him, but looking back on it I think I liked his advice because if he gave me advice he knew what was going on in my life and I knew he cared which made me happy. 

Is there a name for it when a person starts valuing themselves on others like that? When a friendship turns like that? 

I have been reading up on codependency and some of it fits but not all of it, so I was wondering if there's something else it could be? In the line of a mental or social disorder? Thanks in advance x

I wouldn't call it a codependency. And there's nothing strange about it. You need advice, he knows you well and gives you good advice that works for you. Maybe he has become your mentor?

http://en.wikipedia.org/wiki/Mentor

This is the source of the modern use of the word mentor: a trusted friend, counselor or teacher, usually a more experienced person. Some professions have "mentoring programs" in which newcomers are paired with more experienced people, who advise them and serve as examples as they advance. Schools sometimes offer mentoring programs to new students, or students having difficulties.

If there's nothing strange about it why'd he decide to walk out on me and refuse to have anything to do with me?
He was my mentor for a short time but not officially. In my church we have a very strict mentoring system and I have a mentor that is not him.

How old is he, and how old are you?

Maybe he decided the age range was too great and that you are, indeed, becoming too dependent on him, not for just his advice, but in a clingy, puppy-love way. And yes, then that would be a sort of codependency. There's something more though -- let me think on what it is called.

I am 14 and 15 in October; he is 19 and will be 20 in January

It's more the fact I value myself on the way he treats me and responds to me and that I get down if he won't tell me things/if I can't help him that concerns me and probably him.

Just because officially there is a rule in your church concerning mentoring, doesn't mean you don't look up to this older guy, and maybe you have a crush on him, but you know you are to young, but still want him in your life.

See him like a big brother, and you are his little sister, which is probably the way he sees you. But give him some space and not make too much of your life revolve around him, as you are doing now, because you are a bit to emotionally attached for this to really be healthy right now.