Is 70 IQ bad for a 12 year old?

IQ scores are normalized by age, thus theoretically if you got an IQ score of 70 you should always get an IQ score of 70 throughout your life whenever tested. In actual practice deviations in score of about ±5 points are not considered statistically significant. However remember that the only thing an IQ score has ever been shown to consistently predict both reliably and accurately is the ability to score high on IQ tests.