Meaning of "4歳と14歳で、生きようと思った"

Question

I'm trying to breakdown the tagline for the movie 火垂るの墓 in a movie poster: "4歳と14歳で、生きようと思った"

4歳と14歳で

My first guess was that the で here is the て-form of the copula だ, so it just joins that clause to the next.

But the whole sentence makes more sense if it's the particle で that expresses a cause, as in, "since they were (only) 4 and 14 years old, ...", but it seems like a strange place to put a comma.

生きようと思った

The structure [volitional + と思う] here, as I understand from the Dictionary of Basic Japanese Grammar (p.569), indicates the speaker's desire, or what the speaker intends to do.

But I'm not sure how to map that explanation to this sentence.

My best guess here is that the tagline means, "Since they were (only) 4 and 14, I thought that they would live", but I feel like I've misunderstood this sentence.

There are a few other threads about ~おうと思う, but I don't think they answered my question.

Thanks for the help.

Answer 1

で can mark a cause/reason (eg 戦争で多くの人が死んだ), but "to think we will survive because we are 4 and 14" doesn't make sense to me.

I feel this で is the same で as in 1人で生きる ("to live alone"), 全員で考える ("to think together"), 個人で検討する ("to investigate individually"), etc. で marks the (number of) people involved in the action. Broadly speaking, it's referred to as an amount marker here.

Note that ～歳で usually means "at the age of ～" (eg 25歳で結婚する "to marry at the age of 25"). However 4歳と14歳で生きよう refuses this interpretation, and people would imagine something like 14歳(の少年)と4歳(の少女)の2人で生きよう when seeing it, and understand how isolated they are. The catchphrase has been abbreviated aggressively because it's a catchphrase, so you don't have to think it is a common regular usage of 歳で.