I'm 28 and singular-they was taught to me in school; year 2 of English as I said. This is in France which doesn't even have a great track record in producing anglophones.
In French, there is a gender-neutral He and I can assure you you it is nothing like the English one. We quite literally have two very distinct and incompatible "il" ("he") words; the neutral one better translates to "it". (Il pleut => He is raining [sic] => It is raining).
In unknowns, French genders to masculine by default, though some people may choose to disambiguate if targeting a group of people (Same as saying 'He or she' in English).
French has plurals for both "he" and "she" ('hes' and 'shes' => "ils", "elles"). In English, you'd translate both to "they", because you simply do not have plural gender pronouns to work with. "Ils et elles" => "They".
Translating back to french without the disambiguation, we use "Ils". But again, not the masculine one, the distinctly-neutral-albeit-grammatically-masculine-because-french-has-no-neutral-gender one. Which is simply the plural of "it". However, because it's dehumanizing/objectifying to refer to a person as "it", English uses "they".
So there you go:
- Je / Tu / Il / Elle / (Il|Il) => I / You / He / She / (It|They)
- Nous / Vous / Ils / Elles / Ils => We / You / They / They / They.
In English, this is all honestly very instinctive. There are only seven distinct pronouns! But that doesn't mean anglophones don't get to think about how to use them in all different circumstances… "they" is a multi-use tool, just like "you".