You can join any Grand Company at 22, however, you are not bound to a Grand Company until 25.
There is a Grand Company quest (different one for each GC) that requires you complete ALL entry level Grand Company Quests up to the one where you choose your GC at 25.
Once you have completed them all, then Selected the Grand Company you will be bound to, you can get your Choco Whistle.
And while people may complain it's a lot of running around, I find it to be a lot more fun and interesting than feeding a sick Chocobo for a freaking day!
And it gives you a chance to look at the Grand Companies, and see what they have to offer.
And personally I wish I had spent all my GC Seals from the ones I locked out, since you can potentially never get them back (and if you can at some point, they are only a couple thousand, so they are not even *that* impressive).
If you have time to mess around with the GCs, and want to just rake in the seals before leaving (and well, have a 45+ job) you can even get in a run of Into the Dark for each town before picking your GC. Good exp and seals, pick up some crafting gear or what-not...
But, once you choose your GC, you are stuck with it, and THEN you get your whistle.
And, as an incentive I assume, you can't get your Goobue until you are 30 (but it is not locked to a GC).