Timothy Brisby is 6 years old (by human age) and is the youngest son of the Brisby family. By what fans from thornvalley.com and etc. have said about Timothy, he has his father's intelligence and appearance.
Role in Film
Timothy is the youngest son of the Brisby family who was sick with pneumonia and was seen only a few times in the film.
In the sequel, he's chosen to be lead the rats out of NIMH and the main protagonist.
Although Mrs. Brisby was the protagonist of the first movie, Timmy assumed the role of the protagonist in in the sequel, because he had bigger plans than anyone else. Plus, Brisby's role in the sequel is small.
Unlike the rest of his family, Timothy doesn't have an eye color. However, in the "sequel", he has brown irises (just like Cynthia and Martin).