For IRS purposes, "disabled" means unable to perform gainful work, due to a condition that is permanent, or will last at least one year, or will lead to death. Because the focus is on the ability to work, many people with medically recognized disabilities are not considered disabled on their tax return.
In any case, there are no additional tax benefits you can claim for being disabled. It affects a few things if you care for someone else who is disabled, and it changes the treatment of your pension if you leave work on disability before full retirement age. But there is nothing extra that will happen if you do or don't check the disabled box at your age. If you receive disability payments, they either are or aren't taxable, depending on who pays, and your other income, and that calculation is not affected by checking the box.
