Once you unlock them, they are available for all jobs. So you can ride a chocobo at lvl 5 if you unlocked your whistle with a 25. I didn't complete the quest myself yet, but based on phase 2 beta, lvl 20 is when you were eligible to get a Chocobo before.
WoW Blackhand-US-Date of Retirement: 9/21/2010... /Sigh
Devari - 90 Rogue 85 DK Druid/Mage/Warrior 70+
FFXI - Shiva "Retired.... Or not? One more try, honest."
Desmar - 65 Sam 36 Mnk 18 Thf 12 War
FFXIV - Devari Garamond - Sargatanas 50 Paladin / Culinarian / Weaver / Armorer
Beta - Devaria Ariadne - Ultros - Pugilist