Schooling seems to predict safety from unemployment

Source: The Economist

The graph indicates that the longer one's education, the less likely one is to be unemployed.  The effect is found in all the countries of the OECD. It suggests that schooling pays off in later employment, which seems reasonable.

I wonder if there are some hidden variables. In advanced developing nations, who are the people above the age of 25 who have less than high school education? I suspect that they include older people, who while still in the workforce may experience age discrimination. They may be immigrants, who again may not be as fully employed in the formal economy for several reasons. They may also be people who due to some disability found schooling difficult, and thus may also find suitable employment difficult to find.

All these things would make it difficult to infer the value of schooling, disentangling it from the values of being young, healthy and native born.