I would like to ask for your help! If I replace my senior team's coach with a higher-rated, stronger coach, does it also affect the youth team? In other words, are the youth players trained by the same coach as the senior team, and will the youth players also develop faster with the stronger coach, not just the seniors? I couldn't find a description of this in the rulebook. Thank you for your help!

The players get just better trained. Your players and youth stay the same but get better and faster trained. Dont worry.

Thanks

Yes mate, the young players get an even bigger skill boost from better coaches due to their age.

You need to ''assign'' coaches to each player under Training Regime( use specialized training not general), 1st team and youths, in order to have a training boost from better coaches. In other words in the training page, if any player, 1st team or youths, train under general training regime they will not get any boost from coaches 

Thank you, everyone!"