You reach age 70½ on the date that is 6 calendar months after your 70th birthday.
Example: You are retired and your 70th birthday was June 30, 2018. You reached age 70½ on December 30, 2018. You must take your first RMD (for 2018) by April 1, 2019. You will take subsequent RMDs on December 31st annually thereafter, as will be discussed below.
Example: You are retired and your 70th birthday was July 1, 2019. You reach age 70½ after December 31, 2019, so you are not required to take a minimum distribution until you reach 72. You reached age 72 on July 1, 2021. You must take your first RMD (for 2021) by April 1, 2022, with subsequent RMDs on December 31st annually thereafter.
