Which relative pronoun do you use in connection with "generation"?
which or who?
I am confused because I have found both versions on the internet but I think one must be more idiomatic for the English language?
In my experience, you can use either one. Use of "who" tends to focus on the generation as individual people, while use of "that" or "which" focuses on the generation as a social or cultural group. The difference is pretty subtle, but here are two sentences I'd use with this sort of distinction in mind:
"Baby boomers are the generation that will have the biggest impact on the long-run viability of the social security system."
"Baby boomers are the generation who were most caught up in and affected by the antiwar movement of the 1960s."