Life doesn't have a set timeline. You may not know where or how to start but, ideally, a person should begin the planning process from the time they become a legal adult (18). Although a complete estate plan may not be necessary at that time, as an adult only you can make decisions regarding your health and your finances. Without proper planning, control of your health and finances would rest in the hands of the Commonwealth.
Usually, however, a person doesn't begin thinking about estate planning until after they start a family. And with children it is especially important to have a plan in place. Such a plan would include designating the people you want to raise your children and the people you want to handle their funds, ensuring their continued well-being until they are grown and on their own.
With no plan the state decides who will raise your children, who will control their funds, and how your estate will be distributed. These "default plans" can never take into account your unique family situation or reflect your own wishes for your children and assets. Simply put, you need to make these decision ahead of time.