So after posting that I googled my idea that Neil's age doesn't make sense if he's Kat's son, and this guy agrees as well, so I'm not alone:
However, the ages and timelines don’t quite match up. As we know, it takes one hour to invert back one hour. With that in mind, Max would have had to have inverted himself at a very young age – the maximum age he could have been would be 15 years old, as he would then need to invert for 15 years to be the 30-year-old Neil we see in Tenet. Also, 15 years of oxygen so he can invert all time... The Protagonist could also keep young Max in one time period by constantly inverting – but that seems like an awful way to grow up
https://www.gamesradar.com/tenet-theories-neil-max-time-inversion/
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