How old is a border collie full grown?

A border collie will attain his/her full height by around 8 months.

Females and neutered males will fill out up to 24 months.

Un-neutered males will fill out in their 3rd year. The head will grow noticeably larger and the frame will fill out with muscle, and in this way they become distinct from neutered males. They may add as much as 20% more weight in the 3rd year.