What's the age a child should learn the Quran

Learning Arabic language to understand Quran is not possible because Quranic Arabic is very different to the Arabic language and also you need hadith, sunnah and tafsir to understand Quran, i don't think a 4 or 5 year old can learn all that but he can easily memorize Quran as Quran itself says that "it has made easy to memorize".