You may use IRS piblication 907 - Tax Highlights for Persons with Disabilities as a main source of information - http://www.irs.gov/pub/irs-pdf/p907.pdf
Publication 524 - http://www.irs.gov/pub/irs-pdf/p524.pdf addresses issues related to Credit for elderly or disabled persons. You are qualified for the credit if you retired on permanent and total disability and have taxable disability income. - page 1
page 4 - Permanent and total disability. You are permanently and totally disabled if you cannot engage in any substantial gainful activity because of your physical or mental condition. A physician must certify that the condition has lasted or can be expected to last continuously for 12 months or more, or that the condition can be expected to result in death.
Substantial gainful activity. Substantial gainful activity is the performance of significant duties over a reasonable period of time while working for pay or profit, or in work generally done for pay or profit. Full-time work (or part-time work done at your employer's convenience) in a competitive work situation for at least the minimum wage conclusively shows that you are able to engage in substantial gainful activity.
Substantial gainful activity is not work you do to take care of yourself or your home. It is not unpaid work on hobbies, institutional therapy or training, school attendance, clubs, social programs, and similar activities. However, doing this kind of work may show that you are able to engage in substantial gainful activity.
The fact that you have not worked for some time is not, of itself, conclusive evidence that you cannot engage in substantial gainful activity.
The following examples illustrate the tests of substantial gainful activity.
Example 1. Trisha, a sales clerk, retired on disability. She is 53 years old and now works as a full-time babysitter for the minimum wage. Even though Trisha is doing different work, she is able to do the duties of her new job in a full-time competitive work situation for the minimum wage. She cannot take the credit because she is able to engage in substantial gainful activity.
Example 2. Tom, a bookkeeper, retired on disability. He is 59 years old and now drives a truck for a charitable organization. He sets his own hours and is not paid. Duties of this nature generally are performed for pay or profit. Some weeks he works 10 hours, and some weeks he works 40 hours. Over the year he averages 20 hours a week. The kind of work and his average hours a week conclusively show that Tom is able to engage in substantial gainful activity. This is true even though Tom is not paid and he sets his own hours. He cannot take the credit.
Example 3. John, who retired on disability, took a job with a former employer on a trial basis. The purpose of the job was to see if John could do the work. The trial period lasted for 6 months during which John was paid the minimum wage. Because of John's disability, he was assigned only light duties of a nonproductive "make-work" nature. The activity was gainful because John was paid at least the minimum wage. But the activity was not substantial because his duties were nonproductive. These facts do not, by themselves, show that John is able to engage in substantial gainful activity.
Example 4. Joan, who retired on disability from a job as a bookkeeper, lives with her sister who manages several motel units. Joan helps her sister for 1 or 2 hours a day by performing duties such as washing dishes, answering phones, registering guests, and bookkeeping. Joan can select the time of day when she feels most fit to work. Work of this nature, performed off and on during the day at Joan's convenience, is not activity of a "substantial and gainful" nature even if she is paid for the work. The performance of these duties does not, of itself, show that Joan is able to engage in substantial gainful activity.
Physician's statement. If you are under age 65, you must have your physician complete a statement certifying that you were permanently and totally disabled on the date you retired. See example of the statement here - http://www.irs.gov/pub/irs-pdf/i1040sr.pdf You do not have to file this statement with your Form 1040 or Form 1040A, but you must keep it for your records. If you got a physician's statement in an earlier year and, due to your continued disabled condition, you were unable to engage in any substantial gainful activity during 2006, you may not need to get another physician's statement for 2006