I'm extremely confused about whether there are any rules to use On-reading or Kun-reading for kanji words.
A good example is 「弱者」vs「若者」. The former one is On-reading 「じゃくしゃ」, while the latter one is Kun-reading 「わかもの」. However I can't notice any significant differences just from the kanjis. Without checking dictionary, I might guess 「若者」 is also pronounced as 「じゃくしゃ」, similar as 「若年」（「じゃくねん」）