Orochimaru: he won't age as long as he can transfer his soul to another body, changing skin just like a snake. Also, phisical attacks don't hurt him he just regenerates from even only one snake of the thousand he has. the con of this is he would need to transfer his soul in the shorter term, and each time he transfer it he is vulnerable to being resisted.
Kakuzu: Using his five hearts he has an indifinite life span, as long as he more than one heart he won't die cuz he just could dispatch it when reach its limit and obtain another from some generous donant
Hidan: he won't die by exterior forces, but if he have to make some ritual regulary or can die of age neither are know. the confirmed thing, is he can eventually for lack of nutrients but only becase nature will descompose his body. the body needs constant amount of nutrient to repair the celular damage it receives every day, thats the difficult of being immortal, when you age the celular damage non-well repaired keeps accumulating as the cells lose their capabilit to divide. In short-wile, Hidan has the opposite immortality than oro and kakuzu; he is immortal to be killed but maybe not for age.
Tobi: he was sayed to vbe immortal because of him saying he is madara, a dude who must be 90 years old if alive and he appear to be relatively young. we see that he isn't madara, so he isn't immortal.
Madara: the above + he was resurrecte with Edo-Tensei. With Edo he is practically a non-living existence, zombie-spectre-as you like to call it so it's incorrect to say an Edo is immortal.
Itachi: same as Madara.
I may add Sasori: he transform his body in a puppet except for his sealed heart, he find some way to mantain his heart in that condition. He may be some kind of immortal in an special way, a mixture of Hidan and Oro immortality in some characteristics.