I would say from my point of view in grammar context, a very high command of JLPT level N3 grammar would be the bare minimum for me. Note that this done not include vocabulary and specific terms (jargon/slang) required to describe say your job.
For elderly, there is another added level of understanding the Japanese dialect(s) assuming that they do not come from the Kantou region. Of course, one can argue that a good command of "standard Japanese language" would be sufficient for interaction.
One last advice I would like to share from one of my Sensei is that conversation and interaction is not about how "well" or "perfect" you say it but is about how "nice" and "fitting" you say it.
I have seen people with very high scores of JLPT but during conversation they are so bad that it is such a turn off.
Interaction is a form of Art. Hope I answered your question!
Good Luck!
Thank you! You answered my question in exactly the right manner I was hoping for. It's one thing to say, "I have passed Nxx" but what does it mean in real conversation terms with natives. My need to learn jp is to be able to communicate with my partner's parents. Not a requirement from her, since we talk in English, but her parents don't speak a word of it.
