What does the following sentence mean?
The younger and longer boys view porn, the more engrained the lessons they learn become.
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From the article you linked:
As Jonny says: ‘Porn brought me to the brink, triggering anxiety, depression and invasive sexual thoughts about every woman I set eyes on.'
According to the article you linked, those are "the lessons learned from viewing porn". Maybe a more clear word is "the effects".
As boys watch porn longer, and as they start younger, the effects of porn become more engrained.
Deeply impressed or instilled. (source)
So, a boy that does not start very young, and does not watch porn very long has these lessons (effects of watching porn) engrained a little bit, but a boy that starts very young, and does watch porn for a long time has these lessons engrained very deeply.