Kids in the Netherlands are on bikes nearly from infancy... certainly from 'toddler-hood'.
From the age of maybe 2 on up, you see kids riding as passengers with their parents... front of bike, back of bike, papoose-style, however you can imagine.
You also see a lot of kids of (I'm guessing) 3-year-old-up age learning to ride bikes on their own... frequently with mom or dad acting as their training wheels... parents with one hand on their own handlebars, one hand on the back or shoulder of a kid just learning to ride.
Along with learning to ride comes learning to take care of your bike... or at least making sure that it is still there next time you want to use it!
Most adult cyclists, at least in the cities, use massive chains and locks -- in some cases, so massive that they must weigh nearly as much as the bike frame itself. It is almost comical.
Kids take the same approach with their wheels: a big lock is the best insurance.