Party pulled a wish spell and summoned a red dragon as a pet and "IMORTAL" need help on immortal rules. need close to actual dnd rules as to how immortality works.

There are two types: one is just you can never die from old age. The other is you cannot die by almost any means - there might be only one specific way to kill them, need to remove or bypass immortality with magic or just flat out immune to all death.

In most cases for D&D it's the former: the being does not ever die from old age.

my thing is he cant be not killed. so there has to be a WOC rule for immortality 

Quote from ChrisMO >>

my thing is he cant be not killed. so there has to be a WOC rule for immortality 

 
There is no defined rule, just what the word means in the English language and what the DM thinks up.

I don't think you want to give your party an invincible dragon pet.  I'd just say it doesn't age.  As already stated there are no actual rules about immortality. 

That's a prettyenerous wish spell! 

It's totally up to you as a DM! There's no rules for it, and even if there were, you can have Wish do whatever you like.

Do whatever seems the most fun for your campaign. <-- general rule for how to adjudicate "Wish"

I would not have granted the party 3 wishes in one (summon Dragon, make it obedient, and immortal). Wishes are allowed to fail if they are beyond the caster's ability. It is the most powerful spell, but it is not an all powerful spell.

There are no official rules for "immortality". It is expressed in case by case game mechanics. Such as reviving in another plane, not dying at 0 hp unless a condition is met, or immunity to certain damage types. There isnt anything official that is designed to be unkillable.

5e rules provide limitations to Wish. One important one being that using Wish to do anything other than duplicate another spell of 8th level or lower causes a severe problems to the caster.

Since this Wish did 3 things (though I would have simply made the spell fail with no effects other than a wasted slot or, in the situation of a granted wish, a wasted Wish attempt), I would make all 3 effects only 1/3 effective and an increase in "stress" upon the castrt. The dragon is unruly. The dragon merely ages slower and has higher resistances but is not unkillable. The dragon is weaker than a dragon of its age as if having long-standing injuries.

It looks like you're trying to undo something that you allowed. That die is cast. You'll be hard-pressed to fix it. The only crack in this is the vague definition of Immortality in 5e unless you already told them a definition of zero chance of dying in any situation. In that latter case, you're kinda stuck.

OK, here you go, obedient hatchling red dragon that never ages.

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If you have a big red, then it can still be Banished and Iron Flasked.