Martina Hingis (L) of Switzerland talks to Jamie Murray of Great Britain during the mixed doubles final match against Hao-Ching Chan of Chinese Taipei and Michael Venus of New Zealand at the 2017 US Open in NY, the United States, Sept. 9, 2017.
The top-seeded partnership, which won their first tournament together at Wimbledon in July, pushed their team record to an unblemished 10-0 with the win over the number three seed.
This was the third time the 24-year-old Chan has failed to clinch a Grand Slam doubles title. "It's been a lot of fun for me and a great opportunity for me to compete with Martina", he said.
Stephens said she met Altidore, who plays for the USA national team, in fifth grade.
"It's been a lot of fun for me", Murray said.
The only younger Grand Slam girls' champions were Martina Hingis, who won the 1993 French Open at 12, and Jennifer Capriati, who was 13 years, 3 months when she won the 1989 French Open.
Murray won his first Grand Slam title playing mixed doubles alongside Jelena Jankovic at Wimbledon in 2007.
Tennis: Michael Venus loses US Open Mixed Doubles final
Andy posted a picture on Instagram showing Murray and Hingis lifting the trophy on TV, accompanied by the message: "Proud little brother".
Stephens made only six unforced errors in the match to 30 for her opponent Keys in the match.
After the match, Murray reiterated his concerns about prize money for this format of the game.
Neither the 15th-seeded Keys nor the unseeded Stephens had ever participated in a Grand Slam title match before Saturday at Flushing Meadows.
"People talk about wanting to have more players in tennis making more money, not just the very top guys".
"Doubles is a huge part of tennis".