And thanks for the info about YouTube. It's give me a bit of insight.
What cholzhauer told you is correct for now. However at the beginning of the year it was announced that this is going to change on the 6th of June. Starting that day they will return AAAA records to everybody regardless of which DNS server they are using.
Something interesting I've found out.. As someone who uses Google's DNS servers for two reasons (highly reliable, and super easy to remember the address!), I don't exactly want to switch to HE's. I did discover something though.. Their IPv4 DNS doesn't return IPv6 addresses for YouTube, but their IPv6 DNS does!
Interesting. I never noticed that. I am wondering if that is a recent change. Google Public DNS only recently got support for querying authoritative DNS servers over IPv6, maybe the behaviour that you mention was introduced at the same time.
I verified that I see the same behaviour that you mentioned when doing lookups of google.com. I noticed something else. You only get AAAA replies for google.com if you send the query using a native IPv6 address. If the DNS server get the AAAA request from a 6to4 or Teredo address, it will not return any AAAA records.
Sadly, my computer seems to be preferring IPv4 over IPv6 though.
There are many possible reasons why that may happen. It may depend on the type of addresses. Some browsers and operating systems will treat 6to4 addresses differently from native addresses. Unfortunately many of them don't have sensible behaviour in the presence of Teredo addresses. Teredo is so unreliable it should only be used as the last resort, but some will actually prefer Teredo over 6to4.
I have a test page on http://netiter.dk/test-ipv6
, which will show the order of preference for your particular OS/browser combination. Unfortunately the test is only in Danish at this time, but the table on the page should be understandable anyhow.
Notice that some browsers will not just consider the addresses, but will adapt behaviour according to performance and reliability of the different connections. Thus with such a browser the results may appear inconsistent, and it may look like IPv4 is preferred because it happens to be working better on your particular setup.