From the IRS website.
Beginning date for your first required minimum distribution
IRAs (including SEP and SIMPLE IRAs)
April 1 of the year following the calendar year in which you reach age 70½.
401(k), profit-sharing, 403(b), or other defined contribution plan
Generally, April 1 following the later of the calendar year in which you:
reach age 70½, or
See the chart comparing IRA and defined contribution plan RMDs.
Date that you turn 70½
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, 2013. You reached age 70½ on December 30, 2013. You must take your first RMD (for 2013) by April 1, 2014.
Example: You are retired and your 70th birthday was July 1, 2013. You reached age 70½ on January 1, 2014. You do not have an RMD for 2013. You must take your first RMD (for 2014) by April 1, 2015.