The Man is building a little wooden tractor which he is going to give to The Kid for his 5th birthday, in a few weeks' time.
Fortunately/unfortunately The Kid has already seen the work-in-progress tractor in the garage (it looks cool!), and now asks several times each day when he is going to get the tractor.
Which probably makes it not so surprising that this conversation just occurred in our living room.
The Man: "[The Kid], how old are you now?"
The Kid: "Three!"
The Man: "No, you are four."
The Kid: "No, three!"
The Man: "Four. And when you have your birthday in a few weeks time, how old are you going to be then?"
The Kid: "Tractor!"
Yes, in a few weeks time The Kid is going to be tractor years old.