He knew how to look like he knew what he was doing, but also how to "blend in".

It's easier for a nine year old kid than an adult.

If you fly an appreciable amount, you'll notice that when a family with kids goes through, the boarding passes of the parents are checked against the parents' ID at the podium upon entry to the security area, and the kids walk through that check, since the kids don't have photo ID and the parents are carrying the kids' boarding passes.

He got into a water park the same way - just tag along with a family, and he is assumed to be with that family.

However, he still had to go through the metal detector / body scan in order to get into the terminal.

Then, to get on the plane, he made a similar smooth move to get by a distracted gate agent for the airline, probably by tagging along with a family that was pre-boarding.

But he didn't "bypass security". He was checked by the TSA to the extent anyone else is. The point is that the TSA doesn't check ID of minors against boarding passes.

I had no idea!

The only time anyone on a plane checks your pass against your seat is when there is confusion between passengers about whose seat it is.

There is no requirement to sit in your assigned seat if another seat is empty.

If it is a two-aisle plane, then FA's at the door will take a look at your row letter in order to direct you to the correct aisle, but once you've boarded, nobody gives a hoot where you sit unless there is a dispute.

The only thing different with kids is that they can't sit in an emergency exit row.

And, again, with families or groups, they usually work out their seating among themselves. What probably tripped the FA's trigger was that he was no longer sitting with whomever he drifted in with.

But your OP is about TSA, which certainly doesn't care what happens after you pass the terminal checkpoint.

I read stories about grandma's being groped and kids being hassled and they let this kid on a flight! How fucked up is that?

I'm going to guess you don't fly often.

Millions - millions of people pass through TSA checkpoints every week. Go do something - anything - a million times a week, involving human beings, and then come back and tell me that every interaction went smoothly.

The TSA does their thing to (a) check for weapons or hazardous items and (b) deter people from attempting to bring in weapons or hazardous items. They don't sell the tickets, board the passengers, fly the planes, or serve drinks.

Did the kid have a weapon or hazardous item? No.

Was the kid checked by the TSA for weapons or hazardous items? Yes

Does the TSA check boarding passes for kids against photo ID? No

Are kids NORMALLY carrying a boarding pass? No

Think about that last point. Kids don't normally carry their boarding passes. Normally, their parents do.

All the kid had to do was tag along with a family. Preferably, he got in line between TWO families, so that each one would assume he was with the other.

It's not hard for a smart kid to figure out.