You're right, the range I stated was incorrect, that was the range when they "went active," though the 3 you mention in the first sentence were all at least 7 when conscripted, going by birthdates on Bungie.net and the start of the SIII program (2531). Carter was 11, Emile was 8, and Jun was 7 (and just for reference Kat was 7 when Beta Company was created).
You're still ignoring the main reason anybody has an accent: the people they learned the language from had one. If you take some one from, say, Arkansas, who only knows 1 language and compared their manner of speaking to mine, they would sound differently than I despite both of us only knowing 1 language: English.
And you claim that Cat has a very thick accent, yet we have heard her say a grand total of 1 word.