Someone who was 7 during Katrina at that time is roughly 28 today.

Using the Census ACS age brackets, about 20-ish% of louisiana's population is under 15, and another 20 is between 15 and 29. Everyone 30 and older adds up to the other 60.

So a hair over 60% are were at least 7.

But that's who lives there now not who lived there then. Between 2005 and 2006 the state population dropped by 6% and most of that displaced population never returned - people coming in from elsewhere weren't there for Katrina. So the fraction who were both living there AND old enough to remember it is considerably smaller than 60%.

So like I said, roughly half.

Exactly, humans famously have no memory until the age of 18