How to use and create DTOs is OOP world?

1. What is the right way to create DTOs from business objects?
2. Who should be responsible for creating them? BO/DTO itself from BO/some static factory?
3. Where should they reside in code if I have, f.e. some core library and a specific service API library that I need DTO for? In core library next to BO(which seems incorrect)/in specific library?
4. If I have encapsulated fields in my BO how do DTO grab them? (obviously in case when BO is not responsible for creating DTOs)

As an example assume that I have some Person BO like this:

class Person
{
    private int age;
    public bool isBigEnough => age > 10;
}

I want age to be an internal state of Person but still I need to communicate my BO to some api. Or having private field in my class that I want to send somewhere already means that it should be public?

5. Are there any general considerations of how to use DTOs alongside business classes with encapsulated data?