Garmin Connect account is indeed limited by age. Garmin addresses this question for example in the user manual of Vívofit Jr:
"At least one parent must have a Garmin Connect account to use the vívofit® jr. app. To sign up for a free account, go to www.garminconnect.com/start. Your Garmin Connect account gives you the tools to track, analyze, share, and encourage each other. Record the events of your active lifestyle and participate in family challenges by connecting a compatible activity tracker with your Garmin Connect account."
It means you have to create an account in your name, and it is then in your responsability how you control and survey the acess of your kid to the account.
