Lisa is 8 years old (despite the fact that there have been 3 separate episodes which focused on her 8th birthday), two years younger than her brother Bart. Lisa's mother Marge insisted on choosing her name because her father Homer wanted to call her "Bartzina" (due to his already having a son named "Bart" and being too inept to think of anything else).
It should be noted that specific dates should be treated as very loose canon, given the general "timelessness" of the series.
Lisa is an extremely bright girl, with an I.Q. of 159 (in Smart and Smarter) and is a member of Mensa Springfield. Lisa demonstrates an early intelligence by changing her own diaper as an infant. Her other precocious achievements include constructing an elaborate Lego castle, spelling the word "star" on her letter blocks, and completing a Taj Mahal jigsaw puzzle. When unable to attend school due to a teachers' strike, she suffered a sort of "school withdrawal" and invents a perpetual motion machine, for which Homer chastises her, saying, "In this house, we obey the laws of thermodynamics!" She also organized a student strike upon learning that Principal Skinner was exploiting her, when she was the student body president.