I actually started playing 5th as an eight year old. I really enjoyed it, but was no where near understanding the rules or running it on my own, I more told my uncle and older cousin what I wanted to do based off of my understood abilities and they resolved it for me. (Dice rolls, skill checks, kept track of character sheet, etc.) When I was eleven I started playing a lot more seriously, with friends and owned my own books. My younger sibling started playing (reluctantly) at ten, but they also liked it, and I found that mats make it a whole lot more visual for young players and my siblings, which made it easier for them to be interested.

The son of one of my friends started playing around 8 or 9. I have seen people younger than that play, and it isn’t always great, but I think if they enjoy it and you’re alright leaving most of the rules and denser stuff for when they’re older, it’s fine! I’ve seen 10-16 years olds that can’t play as well as my friend’s son, so it’s all relative.

I think I'd put the answer to the OP's question on a bell curve. I played my first game at 16, and I immediately understood the game. I GM'd my first game the next day. Younger, and I'd have understood less, had a longer run up. And now, much later, I clearly understand less - that is to say, I no longer play that game I played when I was 16. Back then, nothing really mattered. We were playing powerful heroes defeating evil and saving the world, and everything was either a distraction, or a slight dusting of glimmer on top.

Today, I'm all about world building, and lore, and perfect writing, setting the stage, drawing my players into the subtle trap of my story telling. That's not the game. That's my personal creative fetish. So, I've dropped off the far end of the bell curve. And though I might claim I'm  a better GM now, I know this to be functionally untrue.

I started in 1979, at age 14, and became a DM only at 15, been going strong every since.

My group is (now) 50 people in size. Not counting the 15 that just joined us, our group has stayed and played together since 1980, and I am not the oldest (that's someone a year older than I). Our group includes our kids and our grandkids, lol.

In the last 40 years, the youngest folks I have ever had at my table were 7/8 years old. The games we played were, um, flavored for such. For our mainline gams, we generally have a standard of 12 years old to join the "grown up games", but as young as 6 to join whoever is doing the year long "Hardcore Game" campaign.

Yes, that is what it is called. No, none of the "grown-up games" are Hardcore. If you are a parent or grandparent of a 6 to 9 year old, you understand why it is a Hardcore game.

The grown up games, however, often feature or involve topics and subjects that we collectively decided were not appropriate for younger ages. I can report that we are all old people who just don't get it.

my brother was 9 when he started playing and he understood it fine

the first time i played d&d i was doing it w/my brother - i was 9 and he was 6, we both understood it pretty well

It is unfortunate that the OP seems to no longer be active. I would love to know how his games went. His kids would be about 7-10 around now.

Im pretty sure I started playing at 8?