It's too early to say with reliable data. We will definitely know in 2-3 weeks with data from many European countries. In Italy, it looks like the percentage of younger hospitalised people is 12%, but growing.
12% of _COVID ICU patients_ in Italy are below 50 years old. Note that this doesn't let us answer the question of how common are ICU cases among young infected people.
https://www.independent.co.uk/news/world/europe/coronavirus-...