Why not just believe what the Bible says?
Exactly! Jesus said to "Go and make disciples of all nations" (Greek word there for "nations" is ethne or ethnos ... can't remember which of the two... -- but it's from where we get our word "ethnic")
So Jesus says to go & make disciples of all ethnic groups (IoW, he wasn't just saying to target political boundary countries).
What Jesus didn't say is "Go and make disciples of all ethnic groups, except for the youngest members of those groups -- where you arbitrarily draw a line of outreach exclusion from them because of some unauthorized arbitrary age you attach to them."
....And the Bible did say that “whole households” were baptized. So that had to include little babies and small children.