How recent should the English language certificate be to apply for Master thesis? When applying for Master in an European country, English certificates such as CAE, CPE, TOEFL, IELTS are required. Is there a limit about the "age" of the certificate?

I got CPE 5 or 6 years ago. Is it acceptable or a more recent is needed? The programs which I have seen, do not clear that out.
Yes there is an expiration limit on these language tests. 

I believe most tests are valid for 2 years. I know for instance that the TOEFL test has a 2 year expiration date. Unfortunately, I do not have any experience with the CPE test, but I have personally never heard of any standardized test being valid for more than 5 years. 

I would suggest you retake the test. That being said, you should contact the program you are applying to directly to get the most information about what tests are/are not accepted and for how long they are valid after taking them.