I am assuming that your question in referring to real estate, means a real estate agent. There is no perfect answer to this question. The starting of any business for profit generally involves incurring expenses to generate income. I am assuming again that you would be an unincorporated entity filing on a Schedule C. Under these circumstances the best answer is the date you incurred your first expense or if you were lucky generated your first dollar of income.
Best of luck in your new endeavor.
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