You increase the holy exp by fighting holy type Digimon. For example Monzaemon and Renamon in Sheer Valley are holy Digimon.
The 13+ means your patamon needs to have an aptitude, level of at least 13. Aptitude, is the max level of a Digimon. To raise that level you have to degenerate your Digimon. You don't get that ability to further on into the game.
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Maturity has more to do with what types of experiences you've had and what you've learned from them and less to do with how many birthdays you've celebrated.