All myth.

I'm not a neuroscientist but I work with them.

The idea that our brains are more plastic at a young age is long held and widespread misconception.

Truthfully our brains are roughly as plastic our entire lives.

The problems are sociological, not neurological. As stated a few replies up, adults simply don't have the time to do what kids have the time to do.

Look at it this way. Do you really think a toddler learning a language via osmosis will learn it as quickly or as well as an adult spending the same amount of time learning it in a class and through study? Of course not. Within not too long the adult will have a deep mastery, whereas the kid will still be in the phase where they're speaking in fragments with terrible grammar and a super limited vocabulary.

Part of this is that the adult already knows a language.

So let it be a kid who magically knows english and goes to a new country and learns a second language through osmosis. The adult still wins.

I started learning chess at age 28. My nephew started at the same time and he was six. I was learning because I wanted something to do with him. Of course I progressed far more rapidly and now at 32, I'm 2250 FIDE. I have not put in as much effort as I could. Not even by half. If I had, I'm sure I'd be a titled player.

My nephew is ten, and has played more than me, consistently, throughout these four years. I crush him at every time control.

I've even heard people say things like "well ok adults have equal plasticity but our skulls are full and that pressure inhibits neurogenesis"

More pseudo-science. Our skulls are not under enough pressure to in any way inhibit neurogenesis. Our brains reach a size and complexity commensurate with what we do in our lives. If you had the time to study a wide variety of things, your total brain mass increases. Every study has shown that intense study over long periods of time, even stressful inevitably breakdown causing study has resulted in increases in brain mass on the order of 100 cc's. And never resulting in any kind of skull pressure beyond what it was at the start.

It's not only possible for adults to learn skills as fast and as well as kids, some adults actually have the time to do it. I've been lucky to have a good amount of time to pursue study in the past six years. I've come a long way in chess in the last four and learned two foreign languages.

I'm nothing special. I have a good memory, but again, nothing that can't be trained in most people with a proper and regular course of usage.

When people talk about the "critical years" (Nigel Short used this phrase in one of the post mortem wrapups during his recent match with Kasparov) they are unknowingly referring to a period in people's lives where they have very little in the way of serious repetitive responsibilities to intrude on their chosen interests.

tl;dr - You can start chess at any age and you will progress as well as but probably better than most kids given the same amount of time, if you have as much time as they do to study it.