We don't know

The richest source of detail concerning Han Tzu is Orson Scott Card's short story "Cheater", which first appeared in his InterGalactic Medicine Show e-zine. The story expores Han Tzu's back story and early life. (This is probably the "spin-off" you are referring to.)

I've read the whole story and it doesn't give his age (or the age at which he is drafted into Battle School), only mentioning that he was older than 5 at the time of the events in question. Even if it gave an exact age, it would be very difficult to extrapolate a "current" age, as there do not seem to be references to his age in any of Card's other works or in the film.

So it seems we just don't know.

We don't know, but slightly older than 5 at the start of Ender's Game

The richest source of detail concerning Han Tzu is Orson Scott Card's short story "Cheater", which first appeared in his InterGalactic Medicine Show e-zine. The story expores Han Tzu's back story just prior to Ender's Game. (This is probably the "spin-off" you are referring to.)

I've read the whole story and it doesn't give his age (or the age at which he is drafted into Battle School), only mentioning that he was older than 5 at the time of the events in question, and so I assume he is between 5 and 6 at that time, Even if it gave an exact age, it would be difficult to extrapolate a "current" age without knowing more details.

We don't know

The richest source of detail concerning Han Tzu is Orson Scott Card's short story "Cheater", which first appeared in his InterGalactic Medicine Show e-zine. The story expores Han Tzu's back story and early life. (This is probably the "spin-off" you are referring to.)

I've read the whole story and it doesn't give his age (or the age at which he is drafted into Battle School), only mentioning that he was older than 5. Even if it gave an exact age, it would be very difficult to extrapolate a "current" age, as there do not seem to be references to his age in any other of Card's works or in the film.

So it seems we just don't know.

We don't know

The richest source of detail concerning Han Tzu is Orson Scott Card's short story "Cheater", which first appeared in his InterGalactic Medicine Show e-zine. The story expores Han Tzu's back story and early life. (This is probably the "spin-off" you are referring to.)

I've read the whole story and it doesn't give his age (or the age at which he is drafted into Battle School), only mentioning that he was older than 5 at the time of the events in question. Even if it gave an exact age, it would be very difficult to extrapolate a "current" age, as there do not seem to be references to his age in any of Card's other works or in the film.

So it seems we just don't know.