I am working for a temporary agency at a client location. The agency wants me to fill out a form with my birthdate, SSN, driver's license number and last ten years of addresses, which they will forward to the client. If I don't do this they say they will take me off the contract.

How can they force me to supply information to their client as a condition of employment, not to mention that the disclosure includes information such as age which an employer can't ask?

They can because there is no law prohibiting it.

There is no law, per se, prohibiting an employer from asking about age. It is a really bad idea to do so but it is using the person's age in making an employment decision that is illegal, not asking the question.

If you are not happy with providing the information, you are free to find another employer who does not ask for such info.

Is it possible that this client is a government contractor and some extensive type of background check needs to be run?

The client is a publicly traded telecommunications provider (national but not one of the largest). So they are regulated but not government.

The agency's explanation was that the reason is that the client has become a "national account". There are others working there for the agency who have not been asked to provide the information because they started before the client's status changed.

It leaves open the question of whether this is a "service" provided to all "national accounts" regardless who they are.

Joe, none of that matters. If the client wants to insist upon having that information as a condition of working for them, they can.