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.
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).
Despite the fact that Timmy's existence in the sequel respects the history of NIMH, the character himself is widely panned by a wide part of the fandom - mainly due to the film having him replace the more fan-favorable Mrs. Brisby as the main protagonist, thus making Timmy one of the most unpopular Don Bluth characters. As a result, the Don Bluth filmography still places Brisby as the protagonist instead of Timmy in merchandise, thus explicitly dismissing him from canon.