If a stat has 8 at Level 5, it will equal your level for the late game (90 at Level 90)
If a stat has 13 at Level 5, it will double your level for the late game (180 at Level 90)
If a stat has 18 at Level 5, it will triple your level for the late game (270 at Level 90)
If a stat has 23 at Level 5, it will quadruple your level for the late game (360 at Level 90)

If a stat has 9 or less at Level 5 and a boosting nature, it is actually higher than it appears to be.
If a stat has 9-12 or 19-22 and a hindering nature, it is actually higher than it appears to be.