kohai isn't really used when in 1st or 2nd person, usually it's a mention of some 3rd party that's when the term is used, either that or specifically saying that I am your kohai in <whatever field or school> + yoroshiku onegai shimasu.... that sort.

senpai is a common use word if you don't know the name or do not wish to address the name, or if you are unclear of the title. and it's an easy way to call a "pro" in whatever field, like going to the master carpenter working on your home's renovations, excuse me senpai, can you look at this for me? so, senpai could be used for asking for advice directed at a pro or what you assume to be a pro. even if they're not completely pro, your use of the title makes them feel more special, which is good if you're asking them to do something for you. same word, same use in any Chinese-speaking area as well.

the word literally means someone of a generation(s) before your own. generation in this case refers to anything from age to title to position to anything denoting multiple ranks, but not anyone of family relations, even in-laws, although, father-in-laws of the same professions, love to hear that term, AFAIK...

In a nutshell:

If the person has more experience or looks like they have more experience (Whether it be in life or in whatever field they're in) you call them senpai.

Likewise, if someone has less experience or are less knowledgeable in something, you refer to them as your kouhai.

It's not always about age, although usually the older you are, the more knowledgeable you'll be (Hopefully lol). However, if you're an old man who just entered a company and the guy training you happens to be much younger, you (as an old man) should refer to the young guy as senpai, since he has more experience/knowledge than you. It might be an embarrassing situation, but that's life

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