age limit on cosigner

I don't think there is an age limit, other than having to be 18. The creditor will just look at your grandmother's credit and determine that she has the money to make payments on a loan in case of default.

Fins is correct...if they are at least 18 and creditworthy with the financial means to act as a co-signer.

Reg B (Equal Credit Opportunity Act) specifically includes age in its list of no-no's. Provided a person has the capacity to enter into a binding agreement, then a lender cannot discriminate against a person's age. [They can use age to give more favorable terms. Maybe they give senior citizen's a lower rate - in that case they can use a person's age]

However that doesn't mean the loan is guaranteed either ... it just means they have to evaluate her using the same objective standards as they evaluate every other applicant.