Coco Gauff and Jessica Pegula were born a decade apart, which does not matter so much when it comes to playing tennis together but definitely creates a barrier when the subject is TikTok. “We don’t really notice the age difference until certain conversations come up,” Gauff said. “It’s definitely a generation divide.” Joked Pegula: “She keeps me young.” The 28-year-old Pegula is No. 3 in the singles rankings, the 18-year-old Gauff is No. 4, and the two Americans are making their WTA Finals debuts in both singles and doubles this week. No other doubles pairing also has participated in the…