If one of my parents has care of me under a parenting order, when does the parenting order cease?

At 16, except in special circumstances. At this time, a Family Court care order finishes. The court will not normally make an order for your day-to-day care if you are over 16.
(s50 Care of Children Act 2004)

What happens if I don’t agree with a decision my parents or guardians are making about my life?

Once you are 16 you can apply to the Family Court and the Judge can change your parents’ decision if the judge does not think it is the right decision. You can only apply to the court if you are challenging your parents’ decision about something important.

If you have been involved with Child Youth and Family Services, and the Family Court appointed a guardian for your care and protection, you can apply to the court from the age of 14 years. Again, the decision you are challenging must be an important one.
(s46 Care of Children Act 2004; s116 Children Young Persons and their Families Act 1989)