What's the meaning of the Mexican expression "se te va el avión"?
Example:
Te lo dije tres veces y de todos modos no lo hiciste. A ti ya se te va el avión.
¿No te acordaste de tu cumpleaños? La verdad es que a ti ya se te va el avión.
In Mexico, it's a way to say that you're frequently forgetful (as in the plane you were supposed to take left without you because you forgot), therefore it makes sense in context with your example phrases, in both cases "se te va el avión" means colloquially that the person frequently forgets things (something told multiple times, and someone's birthday).
"se te va el avión" doesn't just mean that some person is frequently forgetting something, but also if you say stupid/crazy things that are not related to the topic at hand. For example, if I ask you about the weather, but you respond "it was a busy Monday", then I others could reply to Richard "se le va el avión".
Another similar phrase is "dar el avión" (they are not interchangeable) This means that some people do not consider something important that another person is saying. For example, "Mike me dió el avión", that means Mike is ignoring what I am saying or is not paying attention.
Just as a sidenote, let me tell you that in Colombia the expression
"Te dejo el tren"
is frequently used to mean that time has passed for you, and the precise context in which used is related to a person's marital status.
If to or from somebody is said
Lo dejo el tren
it means that for that person that the probabilities of getting married are low, mainly because time has passed and is getting old.
Keep in mind that this expression has a wide array on less educated populations where getting married and early age and giving birth to child is a very important issue and not getting married is seem as a woman's or man's failure.
It is a way to say:
You're running out of time.
Avión is used as much as train, just because those are transports.
