You're right, we're not all equal and some pick things up quicker than others. But with kids starting school they should surely know the alphabet and the phonetics, so they arrive at least having a basic knowledge.
I'm no expert, I'm no teacher, but I made a good effort from 18 months ish to...
I told my son I would drown him if he couldn't read by the age of 3. He couldn't. But I didn't have the heart to follow it through so I gave him another year, and by 4 he could read. And can still read now. Voila.