It is often used generally in all kinds of texts for both children and adults. However, only for words that are especially hard to read or stuff like names, where multiple readings are possible. And many times only the first occurrence of a particular word in a text has the reading as furigana.
There are generally no strict rules, however. Furigana can be used as a tool to aid the understanding of a text, but also as a way of expression itself. (For an example look at the official song text for 紅蓮の弓矢.)
Many manga aimed at children and teens put kanji on literally every word just to make the reading experience as easy as possible. Even words that are learned in first grade will have furigana. It's not because they actually think someone trying to read it might not be able to read that word, they just have it as the standard they chose to put furigana on everything regardless of difficulty.
Woops, meant "put furigana on literally every kanji"
Use of furigana
Curiosity question: Is the use of furigana in Japan mainly for students, both first-language and second-language, or is it often used outside the classroom?