When Private Account is turned on, new followers must be approved before they can follow you. If it is turned off, people can follow you immediately.
Hide Age removes your age from your public profile. This only affects age display on your profile page.
Followers list controls who can open the list of people following you from your profile. You can choose Everyone or Friends.
Friends means mutual follows only. The owner of the account can always view their own list.
Following list controls who can open the list of people you follow. You can choose Everyone, Friends, or Only Me.
Only Me hides the following list from everyone else, while still allowing you to view it from your own profile.
Photos controls who can view your uploaded photos. You can choose Everyone, Followers, or Friends.
Followers means approved followers can view your photos. Friends means only mutual follows can view them.
If you turn on a private account, it is a good idea to review your followers list, following list, and photo visibility so they match how private you want your profile to be.