"It's elite," Rockies manager Walt Weiss said. "He's doing big-time stuff his first few years in the league. He's special."
Arenado, who turned 25 on April 16, tied the Nationals' Bryce Harper last season for the National League home run title with 42 homers.
Third baseman Eddie Mathews had three seasons of 40 or more homers before age 26 (1953-55), while Troy Glaus had two (2000-01). Arenado now has two (2015-16).
Thomas Harding has covered the Rockies since 2000, and for MLB.com since 2002. Follow him on Twitter @harding_at_mlb, listen to podcasts and like his Facebook page.