On the WJIII Achievement, you can obtain Broad Reading scores at age 6.5, Broad Math at 7.5, and Broad Written Language at 8.0. Those are the minimum ages for the respective fluency measures, but not necessarily the recommended ages for reliability.
On the WJIV Achievement, I believe the minimum age is 7 across the board, for a similar reason.
4 and 5 year-olds can score very high, but cannot obtain Broad Reading/Math/Written Language scores. Instead, they obtain Brief Reading/Mathematics/Written Language (WJIII) or just Reading/Mathematics/Written Language (WJIV) composite scores.
