As you grow up, you realize your philosophical views don't apply much to/in your daily life.
Which option is more grammatically correct?
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Grammatically either preposition is fine. Arguably there could be a slight difference in emphasis with other referents, but I can't really see how that would apply in the case of daily life. You might be able to imagine the potential difference more easily with, say,...
These regulations don't apply to/in the UK
...where to implies the regulations apply to the UK as an entity, but in implies the regulations apply to people or circumstances arising within the UK.