Could i have developed chiari malformation type 1 after the age of 20?

Probably not: Technically, most chiari i malformations occur during fetal development. Many people who have a mild chiari i malformation may never have any symptoms or symptoms may develop later in life, but the actual malformation was probably already there. In some unusual cases (e.g., over-drainage of spinal fluid) a type of acquired chiari malformation can develop, but this is less common.

Developmental: Chiari malformations are developemtnal abnormalities and not something that develops over time.