The science of balancing a bike

There is not scientific understanding of
why do people feel comfortable on bicycle
and why they do not when they do not

How do you design a certain bicycle for certain person
or more like
for a certain population of individuals like old folks or kids or
young adults athletes?

How do they should be different to make those population feel comfortable?


https://www.youtube.com/watch?v=HcYbn_RCLiw&