Every skill has a "normal range".
My older brother has the highest IQ in our family and is now a manager for a computer software company. But he didn't learn to read until he was 6. I learned to read at 4... so it had nothing to do with parenting.
Some kids just aren't interested as early, but once they get it, they master it even faster.
If there's a developmental concern, you can ask your pediatrician what is normal. But every kid learns at their own pace.