score:9
The front of my old passport (which I had to change early because of USDHS new rules) was pretty much unreadable like yours for several years, but immigration seemed unconcerned about it for the places I went to. I think they are probably much more concerned about the condition of the inside of the passport than the outside, so if your ID page is still intact, and all your visas and stamps are still legible, I don't think you will have a problem.
That said, the bigger issue often with older passports is that the photo might not look like you any more. I got mine when I was 17 and was still using it at 26, so that did get me a few hard stares from immigration officers (more often when I was returning home to the UK for some reason).
Upvote:1
I used to regularly cross a particular border where the guards would check whether they could peel the laminate on the photo page. Eventually this tampering - by them - led to the laminate lifting as high as the corner of the photo. At another border, this caused me some difficulty, after which - only a few months early - I got a new passport. These days, for my country at least, the photo is printed (and more than once).
Upvote:4
I seriously doubt there exists an international standard for what counts as "damaged passport". Surely missing or torn pages and any damage to the machine readable strip rendering it not-machine-readable will make it damaged but beyond that? I am afraid this falls under "I know it when I see it".
The UK Government has a document on it, though;
A damaged passport is one which is not in a condition to be accepted as proof of identity. Damage may include the following:
but note even this is not an exhaustive listing, it has the word "may" and in general, the first sentence is the key.
Here's what the United States says has on the matter:
The passport has been materially changed in physical appearance or composition, or contains a damaged, defective or otherwise nonfunctioning chip, or includes unauthorized changes, obliterations, entries or photographs, or has observable wear or tear that renders it unfit for use as a travel document, and the Department either takes possession of the passport or sends a written notice to the bearer.