Notice that the second one is actually ものなのかな, thus, they are different, as same as か and かな are different.
In that sentence, ものなのか shows the speaker's surprise, and ものなのかな implies their understanding/consent to it.
Thank you again.
So can we use ものだ or ものか to replace the first ものなのか in the first sentence?
If we can, what is the difference between the three versions?
ものだ is for assertion, so it's different from the original.
ものか can have a similar meaning, but it loses the explanatory tone given by の. Plus, it could be misunderstood as a rhetorical question (e.g. 彼がそんなことを言うものか (=彼はそんなことは言わない)).
Thank you again.
So please let me be more specific, what is the function and meaning of the two ものs respectively? I think the first もの indicate the speaker's surprise while the second もの refers to 歳を取る.
This もの is not a pronoun to represent something. ~ものだ is a set phrase, and those two expressions in the quote are both variations of it, as I already wrote in the post #5. As for the meaning, I already gave it in the post #2, too.
It's just the typical use of という as a quotative. It's the difference between saying "getting older is X" and (literally) "this thing we call getting older" or what-have-you.
It's more of a rhetorical flourish than anything that fundamentally changes the meaning or nuance of what's being said.