The way people talk about it, it's a wonder the OHL graduates any players at all. After all, they spend 3+ years in a row playing against children.
There is a difference on certain age.
Like, in Finland rare of 16-year olds will play against men. That's even vs. Canadian juniors. 16-year olds will play against 16-17--18-19-20-21 aged juniors in U20 level also in Finland. Only "generational" Europeans have succeeded in Finnish/Swedish men's league as 16-year olds. I think these are Barkov and Dahlin. Laine played, but struggled.
As 17-year-olds, there has been many lately. Barkov, Laine, Puljujärvi, Kotkaniemi, Aho, Kakko, Heinola, Kupari, Vaakanainen, Heiskanen, Rantanen, Ristolainen. Season before the NHL Entry draft they start playing against men, practising with men and learning from men.
This is the key season, which will give them some pro taste before any canadian junior leaguer can get to same kind environment. Seider did same in Germany.
It wasn't like this before. Maybe 5 years ago situation changed when Jokerit moved to KHL. Liiga level regressed. Also Espoo Team went bankrupt. So it's kind of coincidence, how the Liiga level of Finland has dropped down, because so many plays in KHL, SEL, better leagues. And some lesser organizations were promoted to Liiga, and they had to find cheaper solutions from kids than Espoo and Jokerit did with their highest payrolls.
It has opened spots for those kids, who after this drop are enough good to play against pros, but at same time some experienced vets still stay on the league, which would have retired in a tougher league, or would play on the 2nd level. They are there mentoring these kids daily basis and it's the biggest factor why Finnish juniors are winning these U18, U20 tournaments, because they are just more educated.