At what point is a textbook considered 'too old'?

I just got my hands on a Japanese textbook, but it was published in 1986 (iirc). It's not that big of a deal since it's a grammar textbook and Japanese grammar hasn't changed too much, but that got me wondering about how old textbooks can be before they're considered an old fashioned way to speak... like if someone who wan'ts to learn Spanish picked up a Chinese textbook from, say... 1910, would they sound old to Chinese speakers? Sorry if this is getting a little confusing, I'm not good at expressing myself.

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