You're totally right, rolig (and in fact, I learned octogenarian "thanks" to my google search for octuagenarian and my discovery that only Italians seem to use it). After that, I found both words in Latin; so I proposed octuagenarian here as a possible "justifiably pedantic" alternative spelling.
According to my dictionary, octogenarian derives from the Latin word for "eighty": octoginta; septuagenarian comes from the Latin septuaginta ("seventy"); sexagenarian from the Latin sexaginta ("sixty"), and so on. So it seems that the numerical prefix used is based on the one that is used with the Latin cardinal number. As far as I can tell from a quick Google and Google Book search, octuagenarian is used mainly by Italians writing in English. I don't think it's standard among native speakers, not even the pedantic ones.