For some context:
In sociolinguistics, a T–V distinction (from the Latin pronouns tu and vos) is a contrast, within one language, between second-person pronouns that are specialized for varying levels of politeness, social distance, courtesy, familiarity, age or insult toward the addressee.
So for example in French there is "tu" and "vous", in Spanish there is "tú" and "usted", etc.
In general one uses "tu" (I'm going to use French as an example throughout) for peers or friends, and "vous" for strictly less or more senior people (so for example a professor would use "vous" with their students and vice versa). As a general rule of thumb, if you would call someone "madam/sir", you would use "vous".
In academia however, interactions are usually more relaxed: it's not unusual to call everyone by their first names, even as a PhD student addressing a professor. Similarly, I would expect that use of "tu" would be more prevalent in academia.
But as a new PhD student, it would have been unthinkable to use "tu" with a senior professor just three months ago, for example. And using someone's first name doesn't necessarily imply that I should use "tu" with them.
Should I use "tu" or "vous" to address more senior people? I guess it's probably safe to use "tu" with other grad students, postdocs or young faculty, but what about other people?
Does the answer change depending on if you see the person face-to-face or if you write them a letter/email?
PS: This is all probably language-dependent. I'm mostly interested for the answer in French, if it can prevent this question as being closed for being "too broad", but if it's possible to answer it in generality that would be great too.