Some young children are allowed to have a phone as early as 6.
どちらも「早い子だと６歳から携帯を持つ。」という言い方です。
1) Some kids get a phone as young as 6 years old.
2) Some young children are allowed to have a phone as early as 6.
このように、Some kids/children ~ as young/early as ... という形を使って「早い子だと」を表現できます。
別の例文で見てみると、
Some people come from as far away as Hokkaido.
「遠い人だと北海道から来ています。」
のように、Some people ~ as far as .... で「遠い人だと」と言うことができます。
ご参考になれば幸いです！
日本 
