Currently, you need to have completed base 2.0 A Realm Reborn; in other words have completed 'The Ultimate Weapon' quest, before the ARR riding maps become visible and available to buy with Allied Seals.

Since the ability to fly in ARR areas was added (which currently unlocks at the same time as the maps), I feel that this restriction is no longer needed, and adjusting/removing it so that you can buy them early on would give a quality-of-life boost for people that just want to fool around with alts and grind some seals. Locking them behind your character's lv15 'Envoy' main scenario quest or the lv20 'Sylph Management' quest would be good options, as there are other things that unlock at those 2 points already.