Twig is bent, so is the tree inclined
The meaning is thought to be centered on the idea that early influences have a permanent effect. The early development of a child is based on a mix of nature and nurture. The effects of parental, environmental and peer influences can have a huge impact on a young molding mind. The experiences they go through and lessons they learn can and will shape who they are and how they act later in life.
“As the twig is bent, so grows the tree” was coined in the 18th century by poet Alexander Pope. (Pope, 1734)
"As a tree bends, so shall it grow" is the alternative form of the saying.