Yes, it is taxable income.
As a rule, retirement age is the earliest age you are eligible to receive a pension or annuity from your employer, if you had not been disabled. Most often that is age is 55, but it can vary from one company to another.
If you retire on disability, the payments you receive are taxed as wages until you reach the minimum retirement age.
Once you do reach your minimum retirement age, it considered a taxable pension income. The full amount is taxable unless you have made any after-tax contributions to your retirement plan (reflected in box 5, Form 1099-R).
