The required minimum distributions rules for Individual Retirement Accounts (IRAs) rely on IRS provided tables that use standardized life expectancies based upon your age. When using their tables (IRS tables) you use your age as of the end of the year to which the distribution applies, and the Fair Market Value of all your IRAs as of the beginning of the year.

CCH has an easy to use calculator here:  RMD   When using their calculator, you should enter your beneficiaries age if you have one. Because of the distribution rules that cover what to do when you don't have beneficiary, I recommend that you do have a beneficiary.