Thats such a cute story!!
I think the answer is there is no age requirement to join FIRST. We have a student who is the brother of one of our juniors. I think he's 10 now, and he is probably one of the best CAD designers on the team! The night of his birthday would have been a typical robot build night (during build season), and he had told his parents that he wanted to have his "party" at Robotics... he didnt want to go out to dinner, or do anything else, he wanted to be at robotics
Unfortunately, school was canceled that day... but we hijacked the robot and some tools from the locked school and headed over to Harris with a portion of our team. He said that was the coolest birthday ever!
Another even younger story, when I was in high school one of the engineers had a son the year I joined. When the little boy was 3 years old, he would always tell his dad he wanted to go see the BIG ROBOT!! He would come in, laugh & play with the robot, and then sit on the floor in our little makeshift field room and sort nuts & bolts... at 3 years old!
Kids are amazing... and they definitely brighten your day. Never underestimate them!