The parties to a contract may pretty much always modify it by mutual agreement (short of an agreement to commit a crime, or one specifically prohibited by law), provided that there is consideration for the modification or the contract provides for its own modification. However, when a corporation has a standard agreement (such as an insurance policy), it often includes language saying that only a limited set of authorized persons may modify the agreement, and any purported modification by anyone other than such an authorized person is not valid, and does not change the original agreement.
So if a company's policy included a provision limiting coverage to people over 25, a reservations agent would probably not be authorized to change that, and any attempted change by such a person would not be valid.
However, if you asked the agent if the policy would cover a person aged 21, and were incorrectly told that it did, and if under the circumstances it was reasonable for you to rely on that statement, and if you would have not entered into the transaction had you been correctly informed of the policy's terms, then a court might hold that the company was prohibited by promissory estoppel from denying coverage on that basis. But if you had read the provision with the 25-year-old limit on coverage, and the provision saying that only authorized officers could change the terms, it would probably not be reasonable for you to relay on the agent's statement, and promissory estoppel would not apply.
A ruling applying promissory estoppel would not be automatic, but would depend on the detailed circumstances, and to some extent on the exact law in the local jurisdiction as well.