Are all Summoning animals Immortal in a sense The great toad sage was very young during the time of the sage of six paths (think about that for a second) and yet he is still alive, If I were to calculate his age, it would probably be a couple hundred eons. 

The same thing goes for the great snake sage, who is also known to be quite old and is still a thing in the Boruto-verse.
Does this mean all Summoning animals can live for eternity if they don't die in battle? Or is it just a feat that can be achieved with sage mode.
The summoned animals live for a long period of time with respect to the human world, but that amount of lifespan is usual in their world (like toads in Mount Myoboku).


  Mount Myōboku has records of its population in a list, and the name of a Mount Myōboku toad stays there until it dies.[7]


I found the above in the wiki page of Mount Myoboku. So its safe to assume that toads do die and are not immortal. They just live very long.