When did Mandarin become a dominant language in Taiwan?

Taiwan used to have their own dialect of Taiwanese Hokkien(Min Nan).

1. When and why did Mandarin(Putonghua) become a dominant language in Taiwan?

2. Do the older generations in Taiwan still speak Taiwanese Hokkien better than Mandarin?

3. How well do the younger generations in Taiwan speak Taiwanese Hokkien? Do they learn Taiwanhua at school?