Welcome, amanda218!
aeh is our forum expert on test scores.
That said, I can provide scant information on two points:
1) IQ scores tend to stabilize around age 8.
"Research shows that IQ scores are more stable after age 7, with the scores becoming less variable as children age."
Source: American Psychological Association - apa.org
"While IQ scores are subject to considerable fluctuation in the early years, by age 8 or 9, they tend to stabilize and become more predictive of future cognitive abilities."
Source: The Mismeasure of Man by Stephen Jay Gould (1981)
"The stability of IQ scores improves markedly after the age of 7, with greater consistency in measurements over time."
Source: Psychological Bulletin (1997) - The Long-Term Stability of IQ: A Review of the Literature by Robert L. Deary et al.
"Children’s IQ scores are less stable before age 7, but after this age, they tend to stabilize and offer more reliable predictions of future cognitive abilities."
Source: Intelligence: A Very Short Introduction by Ian J. Deary (2001)2) Different tests do produce different scores.
A few sources have attempted to build equivalency tables between various test instruments.
https://www.hoagiesgifted.org/highly_profoundly.htm
