Certainly no older than 8. The idea that 8-year-olds are in danger while sitting between two strangers on a packed plane is just loony tunes. They're in vastly more danger at school.

And the cheapskate will just lie to get the BE fare then raise a stink when they can't sit together and someone, either airline or another passenger, will make it so they can sit together. Some people just have no scruples and expect others to bend to their will.

I would set the limit at whatever age they are allowed to fly as unaccompanied minors.

Not surprised it's Air France since I googled those words: "seat selection not available traveling with an infant" and in a few seconds it was all about Air France. I would also have expected BA. But here's the thing. You are flying Air France which does not have domestic flights inside the US so why are you making comments on this United thread? This thread is about passengers with kids purchasing Basic Economy and gaming the system by forcing a nice gentleman to give up his aisle seat.

Please enlighten me as to which of my particular assumptions is unwarranted?

1. Middle seats towards the back are the least desirable and last to be selected?
2. BE seats are the last to be assigned, so therefore are likely to be middles?
3. If a family (i.e., two or more people) wish to sit together, then at least one seat must be an aisle or a window?
4. Given that the OP's flight was full, there likely was no way that once BEs began to be assigned, that two or three together was possible?

In any event, how do you think UA assigns seats? If you travel BE, you explicitly give up the right to select seats of your choice -- and give that right to UA -- which is pretty explicit that your seating desires will be addressed only after all other PAX have been cared for. The OP's sister could have chosen a higher fare class, or chosen to pay an extra seating fee, but AFAWK, she declined.

And there may well have been seats available for the family to be seated together when the tickets were bought -- if the OP's sister had chosen to pay the fees for those seats. The options to sit together were likely there (either through buying a higher fare or paying seat fees). The fact that they were declined was a clear indication that the purchaser either didn't think they were worth the price -- or thought they could obtain the benefits without paying the price. I have no doubt that if UA began to force people on the same PNR to sit together, there would be a thread on FT complaining about the practice.

???

I have no idea how UA assigns seats. And judging from your post neither do you.

Actually it does say very clearly that you are not guaranteed seats together.
If for non-basic economy there is no guarantee you will be seated together.

The problem is assumptions. Some people assume they will get a free meal on their flight, that doesn't mean they should get one.

A friend (mother with 6yr old) recently told me it doesn't matter if the GA does not or cannot seat them together because no one will want to sit next to a crying kid, therefore they will always be accommodated once onboard.

Typical, an adult doesn't take the time to do a tiny bit of research, picks a product that doesn't work well for their situation, and it's the airlines fault that things became tough. Then OP, tries to pull up a supposed law to throw in the agents face, is wrong about the law, and definitely doesn't help the situation.

I know this comes off as crude, but I am tired of hearing people spending hundreds of their own money not even taking 5 minutes to research what they are buying, and then blaming everyone else.

If that person thinks that she can routinely get people to give up an aisle seat for a middle instead of activating noise cancelling on their headphones, I have a bridge to sell her.

Well said.

I often book flights 2-3 weeks out, and struggle to find decent seats for myself, let alone my family of 4. But that is my problem, not others. I put a few minutes of research into it and pick flights that will work. I'm sorry that OP's family member, or my cousin for that matter feel they are above everyone. Flights are going out full most of the time in this economy, and lacking status only makes things worse. Lack of preparation shouldn't hurt others, although I'm still a happy guy to help when asked nicely. My guess is this happens more often than reported, but rarely seen on FT. While I love to help families, a thread like this makes me question why I'm so generous.

Again, how can you be sure that I am wrong when you claim not to know anything about what is right?

Your first assumption is faulty. Middle seats toward the back are not the least likely to be selected. On most flights, the least likely to be selected are E+ middles, but on some flights you'll see entire E+ rows available whilst the back of the plane is completely booked.

The reason for this is that UA will automatically assign regular economy passengers seats during the booking process if E- seats are available. You used to be able to skip this, but it's gotten more and more difficult.

It's not the least bit uncommon for regular economy passengers to be scattered in the back while BE passengers get E+.