In this traditional village, girls are considered marriageable at the age of 18.

marriageable 是由名词 marriage（婚姻）加上形容词后缀 -able 构成的形容词。-able 表示"能够...的"，所以 marriageable 的字面意思是"能够结婚的"，引申为"适婚的"、"达到结婚年龄的"。