When does a human brain stop growing in size? and is it possible that human brain can grow much in your late teens?

Growth not the issue: Most of the increase in brain volume occurs before you are even born, and the majority of post-natal growth occurs in the first few years of life. However, brain size isn't as important as synaptic development. A great deal of "fine tuning" goes on long after physical growth has stopped. Most reorganization occurs in childhood, up through late adolescence, but this process never really ends.