What is the cartoon character Bart Simpson's permanent age?

Bartholomew JoJo Simpson is a fictional character in the American animated television series "The Simpsons" and part of the Simpson family. He is voiced by Nancy Cartwright and first appeared on television in "The Tracey Ullman Show" short "Good Night" on April 19, 1987. Cartoonist Matt Groening created and designed Bart while waiting in the lobby of James L. Brooks' office. Groening had been called to pitch a series of shorts based on his comic strip, "Life in Hell", but instead decided to create a new set of characters. While the rest of the characters were named after Groening's family members, Bart's name is an anagram of the word 'brat'.

At ten years old, Bart is the eldest child and only son of Homer and Marge, and the brother of Lisa and Maggie. Bart's most prominent and popular character traits are his mischievousness, rebelliousness and disrespect for authority. During the first two seasons of "The Simpsons", Bart was the show's breakout character and "Bartmania" ensued, spawning Bart Simpson-themed merchandise touting his rebellious attitude and pride at underachieving, which caused many parents and educators to cast him as a bad role model for children.