As 20 is somewhere between top 5% (17-18) and top 1% (22) on the AMC8, it probably means the student will score above average on the AMC10.
The AMC 10 is a
much more difficult test, so the top 1% have a shot at an above average score, but a student with a 17 or 18 will likely score below average. While the AMC 8 is a pre-algebra test (you can solve all the problems without algebra), the AMC 10 tests knowledge through pre-calc and is especially heavy in algebra and geometry (the same is true of the AMC 12). You see many 8th graders do well on the test; the number of seventh graders is smaller. You'll see very, very few sixth grade show up on any of the award lists. This is why I want to see higher scores from students in lower grades. I followed the lead of the MAA who requires higher AMC 10 scores than AMC 12 scores to qualify for the AIME. Here's how they explain their rationale:
There's no rush to take the AMC 10. Starting in 7th or 8th grade gives students the opportunity to make the achievement roll and plenty of time to improve their score if they don't qualify for the AIME the first time around.