Welcome to dutch, where there are more exceptions than rules, and the natives just ignore the rules anyway!

In general, "Je" is by far the most common form. Children use "u" with adult strangers, adults are generally only expected to use it with people in authority positions, but that's becoming more and more rare. It's still polite to use "u" with strangers, but nobody will be very upset if you don't, unless you're addressing a judge, mayor or your boss's boss.

Some people address their grandparents formally, but most don't. It's still considered polite to use it with much older people, like 30+ years older, but hardly will be upset if you don't.

Quite a few companies require customer-facing jobs always use "u", to be respectful, but even that is getting less. My city sends me letters with "jij" nowadays.