Kitty should start as a 15-16 years old mutant. this way her journey on the big screen would feel bigger and more memorable, becoming a huge fan favourite and kinda iconic if Marvel does it right with her.
Starting just like any XM:Apocalypse or New Mutants cast member just wont have the same impact, because its been done too many times and isnt as exciting, the general audience has seen too many "young adult" franchises and Marvel should try something new with certain mutants, imo. (specially with Kitty or Jubilee. Or both) Iceman could be another choice, but I think a female x-men as the youngest would be a great choice. Iceman could be 18-20. The rest adults, of course.