Is Lissandra immortal in terms of age?
In her lore it says "Lissandra knew corrupting the Frostguard would take centuries," I mean someone who can survive centuries has to be immortal right?
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She is. It's stated that she keeps killing her successors and taking their place to hide it.
all Iceborn are never ageing at least i think so i also suspect 
is ice born and has been alive for a long time maybe not as long as lissandra but still very long it would explain how it is possible for him to be strong enough to punch his way under a mountain and survive it falling on him.and if his dragon slay skin is canon (it probs isn't) it would explain how he can kill a dragon as well.Lissandra might be one of the most skilled tacticians since she survived a war that she lost and got revenge on her sister.
According to lissandra, yes she is immortal. This is a monologue that plays when lissandra wins on the Howling Abyss: http://vignette4.wikia.nocookie.net/leagueoflegends/images/d/d7/Lissandra_EasterEgg.ogg/revision/latest?cb=20130419132711&filename=Lissandra_EasterEgg.ogg
"The Howling Abyss. A bottomless crevasse surrounded by an ancient fortress carved into a glacier. No one remembers who built this place. No one knows its purpose. No one, but me. Long before Ashe and Sejuani, before magic shattered the broken peaks, before the desert swallowed Shurima, it was the time of The Three Sisters. We were so young back then; Avarosa, Serylda, and me, Lissandra.
Back then the Frozen Watchers lived here. I do not know where they came from, but I know they were powerful. They gave us gifts, shared their magic, made us immortal. They named us, "Iceborn". In return, we gave them whatever they desired. This mighty fortress, a vast empire, our loyalty, these were a small price to pay. Avarosa did not see it that way. Like a spoiled child she cried out for freedom. She demanded choice. She sowed the seeds of rebellion and betrayal.
This place is where it happened. On one side, the Frozen Watchers, on the other, Avarosa and her Iceborn horde. Many Iceborn died that day, but in the end, the Watchers fell. Avarosa drove them howling into the abyss. I never forgave her. . . I had my revenge — but that is another story."
"The Freljord, a frozen, desolate place. War is coming to the Freljord. The Avarosan, the largest and strongest of the tribes. Led by AsheSquare.png Ashe, they seek peaceful unification. The Winter's Claw, the most warlike and brutal of the tribes. Led by SejuaniSquare.png Sejuani, they think peace is another word for weakness. Sejuani has sworn to purge this weakness. Through war, the Freljord will be purified. Far in the north, the peaceful Frostguard protect Freljord's ancient secrets - or so the world believes. In truth, the Frostguard protect me, Lissandra. While others bicker, we prepare. When the time is right, we will strike. They cannot withstand us, they are only human. And when the war is over, the world, our world, will be reborn - in ice." ― Lissandra "The Freljord, a home to some, a grave to countless more. Lesser clans have scrambled for the right to rule this frozen land for millennia - all have failed. Now, finally three great leaders have emerged to rally the smaller tribes beneath their banners. The storms of war will cloud the sky when their paths cross, but once this battle is over, the people will find just one tribe left, to rule the land." ― Lissandra "We have waited so long for this day, we have been so patient. Now, my children, my Frostguard, it is our time. "I have seen the future, and it is cold. The Watchers return and they are so pleased with you. They will take away your hunger. They will take away your pain. You will be reborn; like me, you will become Iceborn."
"We will take back what is ours. The other tribes won't be able to stop us. They are but snowflakes. And we are an avalanche." ― Lissandra
Right now we can pretty much say that Lissandra is inmortal untill she end dying.