A lot of "high shine" silver arowana grow up looking normal for quite some time. Generally they start off looking just like a nice normal arowana, usually a little bit lighter in colour, and better shine than a normal arowana and then once they reach maturity they go white. It wouldn't be all of a sudden though, you might notice a significant difference in a matter of a few months, but it wouldn't happen over night. I believe it has more to do with age than size though, so if you power fed your arowana and they grew up quick enough that they didn't colour up with age.
However the leucistic trait in some fish may not express itself when the fish is young, or express it's self when the fish is very young then turn back to normal, and then quite quickly they turn white when they reach a certain age. I did the research after my "piedball" trachycorystes catfish turned almost all black after being almost completely white for the first 6 months of having him.
Regardless gorgeous fish, you are very lucky to have them.
JMorash
... Speechless !
