How much writing is involved in algebra?
To properly show work, a lot of writing can be required, sometimes many lines per problem. It's not just the amount of writing, but the type of writing as well, in that it can be seen by students as unnecessary because the steps are obvious.
Another issue with starting the Algebra I, geometry, Algebra II sequence early is that geometry is also writing intensive (proofs) and some young students who are ready for algebra may not be ready for geometry.
My son was cognitively ready for algebra when he was 9, but I decided to wait a year because of the writing issue.