maybe what you are referring to:

https://youtu.be/zDZFcDGpL4U?t=536

which is a delightfully digestible, abridged, version of this talk:

https://www.thersa.org/discover/videos/event-videos/2008/06/...

whose transcript you can find here:

https://www.thersa.org/globalassets/pdfs/videos/2010/10/rsa-...

whose figures are drawn from this book:

    Break Point and Beyond, by George Land

relevant bit:

    So my question to you is what
    percentage of the people tested of the 1,500
    scored at genius level for divergent thinking.
    Now you need to know one more thing about
    them - these were kindergarten children. So
    what do you think? What percentage at genius
    level? 80? 98%. Now the thing about this was
    it was a longitudinal study, so they retested the
    same children five years later aged 8 to 10.
    What do you think? 50? They retested them
    again five years later, ages 13 to 15. You can
    see a trend here can't you?