For applicants in the situation you describe, i.e., visiting their children in the UK, there are no specific requirements as to the age of ANY supporting evidence. The 3 months thing is something that probably vectored from the PBS side where there are rigid age requirements. Best practices, on the other hand, informs us that more credibility is granted to applicants who do their best to make sure their evidence is recent. PLAB applicants, for example, are required to submit an invitation to sit the exam and naturally such invitations have an implicit time fuse on them.
Nobody expects a marriage certificate to be of any specific age, it can be from yesterday. I have even seen a case where the marriage certificate was less than 4 hours old (and the applicant was successful).
Should all documents be at most 3 months old?
There is no specific age limitation for evidence unless it is specified when filling out the application (PLAB applicants) or mentioned in the guidance. In the general sense, it should be recent in order to establish credibility, but again, it's not a stated requirement.
How about the marriage certificate?
A marriage certificate can be any age. Having said that, note that they are entitled to look beyond the formal document to be satisfied that the marriage is genuine and subsisting. So you would advise your parents to include the relevant evidence in addition to their marriage certificate. These are things like joint bank accounts, joint tenancies, and things that a bona fide couple would be expected to have (there are no "trick questions", but knowing what they will look for can be very helpful).
To clarify my question, they have been married for more than 25 years,
but the document is 3 months old.
It's unusual, and a sentence or two explaining the situation might be helpful, but there's nothing in itself to cause a refusal. It's more important to establish that the marriage is genuine and a certificate only accomplishes a part of that.