Answer by taking a quote from the following article:

Brock Edward Lesnar was born on July 12, 1977 in Webster, South Dakota, the son of Stephanie and Richard Lesnar, and grew up on his parents' dairy farm in Webster. He is of German descent. He has two older brothers, Troy and Chad, and a younger sister, Brandi. At age 17, Lesnar joined the Army National Guard, where he was assigned to an office job after his red-green colorblindness was deemed hazardous to his desire to work with explosives.

Lesnar's next defense was against undefeated top contender Cain Velasquez on October 23 at the Honda Center in Anaheim, California at UFC 121. Dana White announced via SportsNation that the UFC would bring back UFC Primetime to hype the fight. Lesnar was defeated by Velasquez for the title by TKO in the first round.  On January 11, 2011, Lesnar was announced as a coach of The Ultimate Fighter Season 13, opposite Junior dos Santos, with the two expected to fight on June 11 at UFC 131, but he was struck with another bout of diverticulitis and had to withdraw from the fight on May 12. He was replaced by Shane Carwin, who ended up losing against dos Santos. Lesnar underwent surgery on May 27 to help battle his problems with diverticulitis. Dana White said that he had a 12-inch piece of his colon removed.  In its May 2011 issue, ESPN's magazine published a story listing the highest paid athlete based on base salary and earnings for the most recent calendar year or most recent season in 30 sports. Lesnar topped the list for MMA at $5.3 million, which included his reported bout salaries and estimated pay-per-view bonuses.  In the summer of 2011, Lesnar announced that he was returning to action, stating, "I feel like a new man. Healthy. Strong. I feel like I used to feel". His return match was scheduled to be at UFC 141 on December 30 in Las Vegas against former Strikeforce heavyweight champion Alistair Overeem. Overeem won the fight by way of technical knockout in the first round. The result of the fight remains controversial, as Overeem tested positive for elevated levels of testosterone prior to his next fight. Lesnar then announced his retirement from MMA, mentioning his struggles with diverticulitis and saying "tonight was the last time you'll see me in the octagon".  Speculation about a return to MMA lasted until March 24, 2015, when Lesnar announced in an interview on SportsCenter that he had re-signed with WWE and officially closed the door on a return to MMA, even though he was offered a deal "ten times more" than what he had made previously in his MMA career. He further elaborated that, while he was training for months for a return to the UFC, he felt "physically great but something was lacking mentally". Lesnar added that "[he's] an older caveman now, so [he] makes smarter caveman decisions" and that he chose to sign with WWE instead of returning to MMA because he could "work part-time with full-time pay".

When did he retire?
Speculation about a return to MMA lasted until March 24, 2015, when Lesnar announced in an interview on SportsCenter that he had re-signed with WWE