You would be incorrect and correct to some extent. scholars have debated on this matter and some have concluded although right in the apparent understanding i.e. he has been a prophet from birth, none of his actions are considered as a sunnah he did as a child/young person to avoid any misconception or understand of actions that could not be validated. For example: A non-sahabi (Munafiq or Mushrik) making a reference to an action he did as a child/young man that does not have a chain from a sahabi in either seeing/hearing that action. Of course then their is the issue of him getting married at 25 to Khadijah (May Allah be pleased with her) which was significant and prominent that most people were aware of it as part of Quresh history (not Islamic history) due to the nature of him marrying a prominent personality. So the debate was whether this act is considered a sunnah or not. Nonetheless not much of the actions of the Prophet (May peace and blessing be upon him) is preserved in the way those actions are conducted during nabuwwat and in the books of Sunnah you will notice that most actions are in reference to the period after his age of 40. It has been a while since I read upon this so would like to get some more clarity if anyone is aware of the scholars opinions on this matter.