Where did Dirk Nowitzki's Cowboys and Rangers fandom show up?
Both items come from NBC 5 Dallas-Fort Worth coverage on November 4, 2013. Dirk Nowitzki tweeted that "Man. It's stressful supporting the cowboys", which makes the Cowboys fandom his own statement. The same outlet's writer described him as "an outspoken fan of the Rangers and the Cowboys" when referring to the Mavericks forward. The Rangers part of that rests on the writer's description, since Nowitzki is not quoted on the team himself, so it is a report and not a confirmation in his words.
The 2013 statements came during his years with the Dallas Mavericks, and both teams play in the same metro area. He said it again in December 2013, which puts the Cowboys talk on two separate occasions that year. Nothing in the material says what he felt about either team before or after 2013.
Does his fandom connect to where he grew up?
Not in any way the record supports. Nowitzki was born in Würzburg, West Germany, on June 19, 1978, and played for DJK Würzburg from 1994 to 1998 before the Milwaukee Bucks took him ninth overall in the 1998 draft. His rights went to Dallas, where he played from 1998 to 2019. The Cowboys and Rangers fandom is tied to the Dallas years, not to his upbringing.
The Cowboys won Super Bowl XXVII in 1993, the year he turned 15, and Super Bowl XXVIII in 1994, when he turned 16. Those are dates only, and nothing links them to his rooting.
Questions people ask
What is Dirk Nowitzki's favorite NFL team?
The Dallas Cowboys, by his own account: he tweeted "Man. It's stressful supporting the cowboys" in November 2013.
What is Dirk Nowitzki's favorite MLB team?
NBC 5 Dallas-Fort Worth described him in November 2013 as an outspoken fan of the Texas Rangers.
What team did Dirk Nowitzki root for growing up?
Nothing here shows which teams he followed as a child in Würzburg; the fandoms in the record date from his Dallas years, around 2013.