They usually have a kid on them.

14 years can be really hard to distinguish from 16 or even 18 year olds.

Is the driver supposed to try and figure out if they get to be more aggressive? I don't think that's the point at all.

In other words, "they usually have a person on them".

The question here is:

Does the presence of the sign make you behave differently when you're the car driver? Will you be more aggressive and/or reckless if they cyclist doesn't have the tag ?

If the answers to both are no, which they should be, then one wonders the need of a sign to begin with, no?

>Does the presence of the sign make you behave differently when you're the car driver?

Yes. When a car passes a cyclist they should leave enough room to pass safely assuming the cyclist continues in a straight line (unless they signal otherwise). A kid may suddenly try to turn left from the right side of the street, meaning they should be given more room and the car should reduce their speed.

The OC says "Drivers should expect younger cyclists to behave unexpected". I don't expect normal cyclists to do 'random' unexpected things. But when I see a 'L' board, I am just more aware. This works exactly like street signs asking how to tread with extra care.

Are you the sort of person that looks at a car and goes "Oh, a BABY ON BOARD, I guess I'll not cut you off"?

They don't. In the Netherlands, drivers are supposed to watch out for cyclists.