It would be less discriminatory as some people age 4 or 5 years faster than others do "for their age". Saying somebody is mature is a huge compliment at young ages so why can't the couple of years ahead ones be subjected to the same scenarios that their brain reflects that they are capable of handling?
IQ correlates strongly with success by about point five and G (general intelligence) by about point eight. Ted's argument that just since IQ tests were originally created to help figure out which child was mentally deficient and who was just lazy, That they thus shouldn't be used to measure a person's mental competence. Well that just doesn't follow, He said himself that they are a way to see how intelligent you are and newer IQ tests have gotten much more accurate now. Having a high IQ not only strongly correlates with G which is your actual intelligence but it also strongly correlates with your ability to think long term. Having a low IQ strongly implies that you're very impulsive and don't nor can even think of the consequences of your actions.
There are no objective standards to evaluate one's maturity because maturity refers to one's ability to make appropriate choices in given contexts. That's entirely subjective. IQ's were created to decide which students needed more or less resources to learn so it's inappropriate for law. Age is a much better estimate.