I think this hasn't been true in a long, long time. The most recent example of major contributions coming from someone in their 20s would be Evariste Galois around the time of the French Revolution.
Peter Scholze received his Fields Medal at 30 for work he carried out through his 20’s. However he is not the youngest recipient: J.P. Serre received his Fields medal at 27.
Before we get too excited about the Fields Medal as an indicator for age of great mathematical achievements, let's remember that it's only awarded to people under 40 for work done earlier, potentially many years earlier.
Teens? No way, not really ever.