I think that curmudgeonly mindset sets in due to your physical deterioration and hormonal changes. You'd be grumpy too simply getting up out of a chair causes you pain.

If life extension reverses / resets aging at the cellular level and you have the endocrine system of a 20 year old , it is likely that one could retain a youthful mindset. I think people overestimate the significance of experience and underestimate how your body's chemicals regulate your mood.