None, actually! An adult ADHD diagnosis depends entirely on the symptoms shown as an adult. The current (DSM-V) criteria for adult ADHD state that an adult must display five of the symptoms in their table (children must display six of the symptoms).

Here’s a screen-capped article with the table of symptoms and more info about the diagnosis:

https://adhd-institute.com/assessment-diagnosis/diagnosis/dsm-5/

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I wrote this post in reply to a question on Quora. See here for more answers:
https://www.quora.com/How-many-symptoms-of-ADHD-must-you-have-shown-as-a-child-to-get-diagnosed-an-adult