Isn't most of the line-ups U20 for Leverkusen too? Eh, I don't know, I wouldn't put too much into one game of pre-season. I remember seeing other clubs perform terribly at friendly games, but can win games comfortably when the season starts. He may be experimenting with the squad to see which player can do this or that. No harm's done even if they lost 20-0 anyway, especially if the players understand the goal of the match is to try and practice new tactics.
They started with 5 senior players and 6 academy players, after Flamengo scored 5 goals, Ten Hag substitutes the players become 10 senior players and one academy player, unfortunately they didn't able to score any goal. 
It might be okay if they lost against equal team, but they lost against academy players who played in lower league.
I think we will see Leverkusen finish below 10th rank or they sacked the manager at middle of season.
https://www.sofascore.com/football/match/flamengo-u20-bayer-04-leverkusen/GdbsELEb