When did Darwin develop his interest in the natural world?

It is likely that Charles Darwin was interested in the natural world from early childhood. Charles Darwin grew up in a small town, Shrewsbury, in rural England and enjoyed walking through the countryside, picking up natural materials such as rocks and minerals. His father was a physician, and Charles studied medicine for a time but disliked it, and then turned to studying for the ministry.
While in seminary studies, his interest expanded. He read extensively in natural history studies and met a botany professor. The recently published account of the German naturalist, Alexander von Humboldt, profoundly affected him. Humboldt had traveled through Latin America. He also read Principles of Geology by Charles Lyell, who challenged theological explanations for Earth's formation.
The botanist John Stevens Henslow was instrumental in helping Darwin with the crucial next step: at age twenty-two he shipped out on the HMS Beagle and then spent five years (1831–1836) as its naturalist, sailing around the world. His stay on the Galapagos Islands was especially influential.
http://darwin-online.org.uk/BeagleLibrary/Beagle_Library_Introduction.htm