What you wrote sounds superficially plausible, but you're overcorrecting.
It is true that the average age of first birth varies widely based on socioeconomic factors, but it's up for all groups. The average age to have a first child was 21 for a woman in the US in 1972. In 2018, it was 26. For women without a college education it was 23.8, but that's still higher than it was in 1972. https://www.nytimes.com/interactive/2018/08/04/upshot/up-bir....
The US is not the most extreme country in this regard either.
So yes, there will probably be fewer great-grandmothers in the future, though of course there still will be some.
2016 looks almost binomial. We also need to take into account population size (significantly more in 2016 compared to 1980). It could be that there are just as many young women having children - but there are now more mothers >30. So maybe there won't be less great grandmothers, just a lesser percent of the population.
It is true that the average age of first birth varies widely based on socioeconomic factors, but it's up for all groups. The average age to have a first child was 21 for a woman in the US in 1972. In 2018, it was 26. For women without a college education it was 23.8, but that's still higher than it was in 1972. https://www.nytimes.com/interactive/2018/08/04/upshot/up-bir....
The US is not the most extreme country in this regard either.
So yes, there will probably be fewer great-grandmothers in the future, though of course there still will be some.