Google announced this one in December and I don’t know how I missed it.
You can now add a three-level reading level filter to your search using a pulldown. Options are: no reading level displayed, annotate results with reading level, show only basic results, show only intermediate results, show only advanced results.
Levels are based on two criteria: vocabulary and sentence structure.
Searchers can also now select the reading level filters under More search tools in the left navigation bar. Also new is a very sweet and handy dictionary feature.