Hi Mike, and welcome to cruise-chat.com.
The Freestyle Dailies for N. Pearl do indeed note that the Bliss Ultra Lounge is, after 9:00 pm, for pax ages 21+. I don't know how they enforce it once the clock strikes 9:00. The other lounges do not have this restriction, as far as I can tell.
Also, if you or your TA haven't done so already, you need to familiarize yourselves with
NCL's age policy, particularly the parental consent letter that may be required for boarding.