You can click on the link in to the mount tooltip in the article. It requires level 40 to ride.
That was not really the question. When can my level 85 buy it? In LK you had to get to Dal to buy the mammoth. In Cata, there were a number of vendors that you had to be 84 and unlock the TH zone to get there. Can a max riding level 85 buy it immediately or are there zones to unlock? And if there are zones, is there a level requirement to them?
grexly75 has the most info - best case you will need to be lock summoned to an 87+ zone.
Nothing definitive re: if phasing will prevent the summons from working or if the vendor will be phased in once you get there. Is there vendor rep/friendship status to get before you can purchase it? Nor if the mount is account-wide or per-toon?
Yeah my account was compromised on Diablo 3.
Same day i was playing, happened like an hour after I logged off.
They banned me completely and I had to go through a ton of junk just to get it fixed and then had to reattach a diff authenticator.