This depends completely upon your family and what their traditions are. Some families like to start early, arund 2 or 3pm. Others do not start until 5 or 6pm. There are also some that may have late Thanksgiving dinners at around 8-10pm. It varies depending on people's schedules.
Start eating or start preparing? Prep for many starts the day before with pie baking and side dish making. Turkey cooking starts the next day and usually is begun based on when you would like to eat your feast and the size of the turkey.