How large of a branch can one air layer?

[Wires_Guy_wires]

I thought I read that older trees don't air-gap very well because after a while the hormone needed to produce roots has diminished.... something like that anyway.

Anyone know anything about that? Maybe it's just oak trees...?

It's just oak trees, and others, that lose the sensitivity to auxins in more developed tissue.
This is a common thing in woody plants and it's a gross generalization since there are exceptions to be found, but it is true in general terms that the older a structure is, the more it can "lose" re-programming functions.

They still require these hormones to function normally and these hormones are still present in all of the plant. But the genetic parts that allow cells to become something else (cambium to root for example) get blocked after so many years of non-use. See it like a tent in your attic, you could make a rain coat out of it. But you'd need to know it's there inside some hidden cardboard box, and you'd need to find it, work it, and then you can wear it.
If the tent was still in your hands, it would be a lot easier. Plants can (not always!) grow like that, and functions can become difficult to access if a hundred years worth of boxes were stacked on top.
The best chance is to use young growth, that hasn't had many boxes filled yet.
Seedlings usually are in a stage where no boxes have been packed and everything is still easy to access. Same goes for meristematic tissue in the buds.

 

[Gabler]

I would add that even young oaks are very reluctant to grow roots above the root collar. Of course, because oaks are so easy to propagate by seed, there is not much reason to do so.

 

[Bonsai Nut]

See it like a tent in your attic, you could make a rain coat out of it. But you'd need to know it's there inside some hidden cardboard box, and you'd need to find it, work it, and then you can wear it.
If the tent was still in your hands, it would be a lot easier. Plants can (not always!) grow like that, and functions can become difficult to access if a hundred years worth of boxes were stacked on top.
The best chance is to use young growth, that hasn't had many boxes filled yet.
Seedlings usually are in a stage where no boxes have been packed and everything is still easy to access. Same goes for meristematic tissue in the buds.

*wants to see what's in @Wires_Guy_wires attic*

 

[Wires_Guy_wires]

*wants to see what's in @Wires_Guy_wires attic*

There are in fact, two army rain coats that can be combined to a tent in a box somewhere.
Whoever designed those either loved the elements, or never slept in one of those tents.