I was referring to the common mental afflictions that chip away at people's cognition in their twilight years. Some of these are probably brought on by physical failings in the brain's chemistry, but some might accrue from the experience of life itself. For example, traumatic experiences, especially during childhood, that lead to emotional or cognitive impairment. I was suggesting memory wipes or false memories as a means by which an immortal person could retain the "normal" mental rhythm of being young, growing old, and dying. I think we will likely figure out how to make our bodies renewable before we figure out how to make our brains do the same thing. For evidence, consider the advanced state of internal medicine today compared to the relatively rudimentary methods of psychological counseling and therapy. With regard to brain plasticity, I may have used a term of art improperly. I was referring to what I believe to be a commonly-held belief that as you get older, your capacity to learn new things decreases. This is why it is so much easier for young children to acquire second and third languages than for an adult. I'm not sure if it decreases forever or stops at some baseline, but based on watching my grandparents with a computer, I'm going to guess it declines forever.
If I had a way out, absolutely. Experience anything I want, with the serenity to know that nothing truly matters. Sounds like nirvana to me.