This coach says, "One other local coach mentioned that parents with children under 9 were
not able to register for an FLL team. I contacted FIRST and was told
that they can be manually added to team rosters and that a paper waiver
form will need to be filled an submitted for these kids at local events."
In Virginia-DC FLL, you have to provide the ages of your team members twice--once on the FIRST website, and once for VA-DC FLL registration. I haven't heard about a lower age cutoff on the VA-DC site.
VA-DC FLL Referee Advisor