If by "teenager" we mean adolescents, then we have to realise that children today develop faster on average mainly because of modern nutrition. No matter where the "teens" start linguistically, their sexual development starts earlier than it did a century ago. That affects many things. An example is the change from primary education to secondary education. In many countries, children go to different schools when they are 11 or 12. The change to single-sex classes is often done at the age of 11. If a country decides to educate girls and boys separately after puberty, then it may be necessary to change schools a year earlier than previously.