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Maybe not really immortal, but to be able to reverse the effects of senescence. Who wouldn't want that ability?

Maybe it would give us more time to figure out how to solve the world's problem.. People would now know they would stick around for a while, so they wouldn't want to mess up Mother Earth

What would be the ideal lifespan?

Lol.. Well, the point is; it should be placed into our hands the ability to determine how long we would live. But that's my opinion though :D