I think it's irrational to complain about social promotion in an educational entirely focused on the fiction that age = some specific learning.

But yes, time is a massive problem. We moved as many of our elementary schools to K-5 multiage learning spaces - and everyone did better. We worked to break down age barriers in secondary too.

For real examples see if your library has Timeless Learning.

https://irasocol.medium.com/undoing-academic-time-d0f95f65ebd7