This is what Google say:
Google guidelines [adwords.google.com]

Some client accounts that you manage may not be eligible to receive credits, depending on a number of factors. In order to apply a credit to a client account, the client account must:

* Be newly created; no more than two weeks old.
* Be linked to and managed through your My Client Centre account.
* Have unique credit card and billing information (i.e. different from yours).
* Be a self-managed account.

I had a client sign up an account today, he set up billing, added an ad, I got his customer ID, I linked it, he accepted... it comes up on the list of accounts to apply credits to [b]but when I try to apply... it says the account isn't eligible.

Can someone explain?