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.
- What does it mean?
- What does it have to do with "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).
It is a way to say: