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.
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.
Low birthrates are a constant in every industrialized country independently of culture.
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.