This is more of a recommendation than a hard rule. However, that said if you don't stick to this rule you will most likely run in to the following problems...
- 5th graders will generally not find this activity challenging enough for their intellectual level, especially if they are mixed in with younger kids
- 1st graders or Kindergarten students will struggle to keep up with their older team members. But more importantly a 3 or 4 hour JFLL Expo will be a lot to ask of kids this age. So even if the shorter meetings work out OK, you may find they will struggle a bit with the length of the Expo itself