I have owned this steel string acoustic for probably around 2 years now, and I've just started playing it more frequently as of late. I haven't changed the strings for at least 6 months so they are pretty dirty and stuff and I'm wondering if that is the cause.

Anyway, the problem is this: playing the strings open and even chords close to the neck sound fine, but as I progress up the neck (especially on the higher strings like G,B,E) the intonation gets way worse. Harmonics on the higher strings have to be played at the 13th fret and notes past the 7th fret sound horrible.

Like I said previously I am wondering if the old and dirty strings are the cause. I've looked around for a while and haven't found any solutions so I'm hoping you guys can shed some light on the subject.
the strings can be a problem, and replacing them would help. But intonation problems like this on acoustics usually mean that you have to remove the saddle from the body and move it to the right location and glue it back on. I suggest you take it to a guitar tech and have them set it up for you because that is some serious work.
Quote by strat0blaster
This is terrible advice. Even worse than the useless dry, sarcastic comment I made.

Quote by Cathbard
I'm too old for the Jim Morrison look now. When I was gigging I had a fine arse.
^ Removing the saddle is easy...you can pull it out with your fingers on most guitars.

It's possible your guitar needs a set-up, and that the saddle is not correctly compensated...it's not as easy to intonate an acoustic guitar as it is an electric.

What kind of guitar is it?

(and old strings could sound dead, but I don't think they would mess with the intonation...)

Partscaster/Tele into a bunch of pedals, a Maz 18 head, and a Z Best cab.
like roamingbard says, sounds to me like your guitar needs to be intonated. that does not necessarily mean you need to move the saddle - the saddle may be shaped wrong and might need to be compensated by a pro.
Quote by Skeet UK
I just looked in my Oxford English Dictionary and under "Acoustic Guitar", there was your Avatar and an email address!
Also, exessively high acttion at the saddle can cause this. Essentially, you pull the strings sharp due to the extra travel.