I was 16 when I first read Spinoza's Ethics. I didn't understand much by then, what I did though radically changed my conception of the world. It's not an easy philosophy. It creates havoc in our conception of human society. No free-will, thus no sin nor merit, for example.

Spinoza is also a great philosopher for people not used to the "canonical" philosophical methodology. The treaty looks and works like a formal mathematical proof with definitions, axioms, and propositions systematically proved by pure logic.