To OP:
The advanced search function in
RV Park Reviews might help narrow down your options a little.
Click on Campground Search in the top banner:
Enter the city name or state you are interested in, making sure only City Name or Region Name is checked in the Search Fields. Then, in the section titled "Filter Results By Amenities", use the pull-down arrow beside "Family Friendly" to enter "NO".
Of course,
RV Park Reviews is not a comprehensive listing of all the facilities available, but a lot of forum members use it to select their destinations.
BTW, from what I've read, most parks advertised as 55+ cater their amenities to their long-term 55+ lessees/lot owners, mainly retirees/snowbirds, and there are tax implications in some states (i.e.no school taxes payable if at least 80% of occupants are over 55).