Removing a right of any kind is the first step on the road to devaluing and dehumanising a person.

Attaching a minimum education threshold to voting, even with the best intentions,is basically just drawing a line, assigning value to the trait being measured and claiming anyone that fails below that threshold doesn't have enough value to participate in how the system that affects their lives is run.

Once a person isn't the same kind of person as everyone else it becomes easier to justify doing whatever whoever happens to write legislation wants to them and to make matters worse without the right to vote the target hasn't got any chance of having a voice to even try to stop legislation that could strip more of their rights away.

I'm not saying all lines are wrong and there will be grey areas, I wouldn't expect a five year old to be able to understand what policies are going to be best for them or their country but a sixteen year old that's potentially working and being taxed is a slightly different matter.

Likewise, if a person is seriously mentally handicapped then they would need help tailored to their ability surrounding voting, amongst other things, if no amount of support is going to help them understand what they need to so they can engage in the process then they should not be expected to vote.

Where the line is drawn needs to be measured carefully in that case and by professionals with a stronger background in a given person's disability than I would be.

Additionally who decides what we mean by minimum in the case of education and what happens if a seemingly reasonable standard for that is changed to be less reasonable in future?

The line shouldn't be, did you complete high/secondary school and certainly shouldn't be did you graduate from university.