I hope any US lady skates late into her 20's if she loves the sport. Some of the best skating has come from ladies who've stayed in for years and made comebacks. Many people kept saying Czisny and Zhang should have retired. They didn't. They set goals and turned their skating around even when it looked hopeless.
I'd like to see Nagasu and Flatt stick it out for years to come. And I expect the same comeback stories for Nagasu or Flatt, even with younger talent coming up the ranks. Younger talent doesn't always work out. And the years of experience between these 2 ladies will only help them.