It's as hard as you want it to be.

Here in Fontainebleau, there are bouldering circuits in the forest set for the level of our three year old. (Seriously. Painted with little pink hearts next to the arrows).

They get used, too. Even for me, climbing pretty hard these days, I'll often run off and do an easy Blue or even easier Orange circuit just for the joy of movement over 40-80 nice problems, few of which would even get a V0 if you translated the grade across.

The difficulty scale goes as far as you want in either direction.