But at age 6 you've already been immersed in the language for 6 years. The problem there is to link it to its written form.

When you learn a second language you have the baggage of the first language hindering you (talking about orthography here), and you normally don't start by speaking the language exclusively for 6 years before learning how to read. If you don't know what the language is supposed to sound like, of course it's hard to understand how to read in it.