...How in the world is a shinigami going to use their "extraordinary power" without training and gaining skills?

I mean, without shikai, a Shinigami has the following methods to fight: Kido, Zanjutsu, Hakudo. Hoho is effectively always a defensive/passive skill.

You can't use kido without training. You can't use Hakudo without training. And you can't use Zanjutsu without training.

Ultimately, they're people with swords and spells (and occasionally fists). Do you know how you don't cut off your own hand when you go fight someone with a sword?

You train.

Now. When all you have is a basic sword, and you come up against a guy with absurd strength and toughness (remember, Chad withstood a falling telephone pole before his powers manifested, as well as uprooting and swinging one) who can shoot blasts that, offhand, look equivalent to full power mid-level Kido....Yeah, you're in trouble.

But this doesn't mean they're chumps who lucked out on the superpower lottery and have no skill. They're probably...average. And Chad is way the heck above average.

I would imagine your average Unseated Shinigami to be roughly equivalent to, oh, a Greek Hoplite. Better than an untrained peasant with a sword, but nothing truly superhuman. The ones who really apply themselves to their training become officers.

As for the age thing...I'm not sure why you hate it. I mean, they're not in the physical Mortal World. Such things would act differently.