έξη and έξι
1)
I can see many έξη in a book like this:
Όταν ακόμη ήμουν έξη χρονών, είδα μια υπέροχη ζωγραφιά σ' ένα βιβλίο για το Άγριο Δάσος, που είχε τον τίτλο «Αληθινές Ιστορίες».

Where έξη was used instead of έξι.
Is it allowed to write έξη instead of έξι? Or is it a typo?

2)
Which is the correct spelling between ζωγραφιά and ζωγραφία ?

Question

έξη and έξι
1)
I can see many έξη in a book like this:
Όταν ακόμη ήμουν έξη χρονών, είδα μια υπέροχη ζωγραφιά σ' ένα βιβλίο για το Άγριο Δάσος, που είχε τον τίτλο «Αληθινές Ιστορίες».

Where έξη was used instead of έξι.
Is it allowed to write έξη instead of έξι? Or is it a typo?

2)
Which is the correct spelling between ζωγραφιά and ζωγραφία ?

Answer

"Έξη" is a word that was used in the ancient language and it comes from the verb "έχω" which means "to have". 
Έξη is a habit that got acquired due to the repetition of an action, or it can mean possession of something. So it has nothing to do with its homophone "έξι". 
Plus, as far as I know we don't use "έξη" in today's language at all.

The correct word is "ζωγραφιά".