This discussion may contain spoilers!
Every one has been debating on whether Clementine was 9, 10, 11 and so on but after the first 10 minutes, you can clearly see that Season 2 started like 6+ months after Season 1 and then another 16 months past. That's at least two years which would make her 11 and match the pre-teen title Telltale gave her. So no more, "she's still 9" stuff. Her age also shows in her voice, maturity and physical appearance.