The RMD rules only apply to tax-deferred retirement accounts. After-tax retirement accounts are exempt from the required minimum distribution rules. So if you have a traditional IRA, you'll need to start taking RMDs when you reach 73. On the other hand, if you have a Roth IRA, you don't have to take any RMDs, no matter how old you get. You can leave your Roth IRA investments alone to grow tax-free for as long as you'd like.
How your IRA RMD is determined each year
Your RMD is calculated each year once you reach age 73 and depends on two main variables -- your account balance and your age.
Using one of two IRS life expectancy tables, you can find the expected distribution period that you'll use to calculate your RMD. Most people use the Uniform Lifetime Table, but if your IRA's sole beneficiary is your spouse who is more than 10 years younger than you, you'll use the Joint Life and Last Survivor Table instead. You can use the IRS' worksheet and reference tables (in Appendix B) to help calculate your RMD.