@DTS Engineer None of the above linked documents answer any of these 4 questions.
Based on https://developer.apple.com/documentation/declaredagerange/requesting-people-share-their-age-range-with-your-app
we can answer questions 1 and 4 ourselves. We can expect that for all users with Brazilian App Store accounts:
Before the date, isEligibleForAgeFeatures will return false.
After the date, isEligibleForAgeFeatures will return true.
For Question 3, the Brazilian law says:
This Law shall enter into force six months after the date of its publication.
And I think it was published on September 17, 2025, so it should be effective March 17, 2026.
I don't know anything about Question 2's answer.