My son just turned 5 a week ago, and I recently got him started playing D&D (BX/LL)!
It came about because I bought myself the WotC re-release of the "Dungeon!" board game last month, but my son loves playing it too, he wants to play it every single day! So one day we instead did a short random dungeon-crawl around the "Dungeon!" board using LL/BX rules. He liked it, so we did a few more over a few days. I did take it a bit easy on him, but he's already got two of his three characters (F, C, MU) up to 2nd level.