when used colloquially "oyaji" often refers to guys who are old enough to have children of age, but not necessarily old enough to be a grandfather or have grey hair or prominent signifiers of age other than "facial hair" (which is perceived in Japan to be a major sign of getting old)
"ossan" typically refers to guys in/around their 30's, "oyaji" in their 40's, and "ojisan" or "jiji" 50+, but this of course is not a rule, more of a common practice
of course, it's important to know that all of these terms are simply derivations of the actual terms for "father", "uncle", "grandfather", etc
おっさん (ossan) from 叔父さん (ojisan; uncle, specifically one younger than the parent in relation)
おやじ (oyaji) from 親父 (oyaji; father, "old man" as in the colloquial use in English) (proper term is "(o)tousan" お父さん)
爺・ジジ (jiji) from 祖父 (jiji; grandfather, "geezer" [specifically the first kanji])
worth mentioning is the "proper" way to refer to one's grandparents, as the above is rather rude and generally only acceptable in such colloquial use, and is NEVER considered a polite way to refer to people directly:
that is, ojii-san/ojii-chan/jii-san/jii-chan are all acceptable ways to refer to one's own grandfather, in descending order of politeness.
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