How old is Bart?

Answer 1

Bart has always been 10 years old, except from the episodes set up in the past or in the future such as "Lisa's Sax" (set in the past) or "Future-Drama" (in the future)

Comments

Bart Simpson is 1 year old in "Lisa First Word" (also 2 years old).
5 years old in "Lisa's Sax".
8 years old in "And Maggie Makes Three".
18 years old in "Future-Drama".
25 years old in "Lisa's Wedding"
40 years old in "Bart to the Future" and "Holidays of Future Passed".