Yeah. Tried to get mine to ride a bike at 3.5 and he kept falling over. Now he tries again at 4.5 and he just immediately drives away.

Indeed, and it varies with each kid. My younger brother and I were 18 months apart. Whether he was "early" or I was "late," what was my mom going to do with him while teaching me to read? So we learned together.

Also, school curricula in the US are all based on grade year... which spans almost a whole year of age within a class based on enrollment date cutoffs.

Exactly. I try things with my son and observe if he's ready or not. There's no rush, nobody will care at 30 if you were a year "late" learning X. For example, I'm very skeptical of the trend of trying to teach advanced/abstract math to kids earlier and earlier. If they're that one in 100 and they are interested, sure. But in your late teens you just have so much more brain power, you'll easily catch up. I remember being intimidated in CS undergrad by the ubernerds, who had been coding since they were 6. But within a few months we were all at the same level anyway.