「弱者」vs「若者」Any rules to use On-reading or Kun-reading


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」）




Sometimes kanji compounds use kun-yomi readings for both characters (instead
of the 'expected' use of on-yomi). This is just a feature of kanji that
learners have to come to terms with. The general guidelines for reading kanji
contain many exceptions.

This has been discussed at length on the site already. See below:

  * [How are the different pronunciations of kanji used, such as onyomi and kunyomi?](https://japanese.stackexchange.com/questions/17696/how-are-the-different-pronunciations-of-kanji-used-such-as-onyomi-and-kunyomi)
  * [General guidelines for choosing 訓読み vs. 音読み in kanji reading](https://japanese.stackexchange.com/questions/43197/general-guidelines-for-choosing-%e8%a8%93%e8%aa%ad%e3%81%bf-vs-%e9%9f%b3%e8%aa%ad%e3%81%bf-in-kanji-reading)
  * [Identifying the correct reading for multiple Kanji](https://japanese.stackexchange.com/questions/78797/identifying-the-correct-reading-for-multiple-kanji)

