The majority of people being old is a relatively modern problem. One could even argue that it is a problem specific to cultures that favor low or negative birthrates.
The list of countries with no low or negative birthrates is very short (mostly heavily underdeveloped countries). That's why the Gerontocracy applies nearly everywhere. I doubt it was favoured, but it's result is the shift of incentives from long term (young people) to short term (old people). Democracy fails here.
No cultures directly favor low or negative birthrates.
But we've known since the 1800s that it follows from female education (and this seems independent of culture, it was first observed in France, but you can see the same trend in any African country, or even Iran), which is favored.
France went through a demographic transition 100+ years earlier than all other European countries and then had a population boom in the 50s and 60s. I doubt an average French women was less educated in 1960 than in the mid 1800s.
I don’t think it’s that straightforward, material conditions in combination with massively lower mortality in all age groups and a shift in social values must be playing a significant part