My now 12.5 years old daughter started to learn English on Duolingo when she was around 9. She didn't complete the full course, but has around 100-150 days of lessons. It definitely helped her starting to understand the language. She loves Harry Potter so much that after reading the whole series in Romanian, she started to read the first volume in English. Didn't finish that either, but managed to read about 100 page…
That's a remarkable jump, to make that claim based on some rough assessment of a single person.
I didn't have duolingo but I still outperformed most of my colleagues who were in extracurricular English. What can I conclude from that?