Answers:
1. Arf and P53 were being injected by a technique which is engineered genetically and it is for this reason which made the mice to continue living longer.
2. No, i don't think people should be able to be engineered to have high levels of the two enzymes unless it is for curing cancer. I don't think anyone should live forever even though I would like too, I don't think it would be a good idea. If everyone started living forever we would be increasing the amount of produce and resources we use up. There for most of the age-less will die from starvation (over-population).
No early death should be allowed to occur in children not unless it is God's will, therefore genetic engineering is much allowed in children.
It is good to stop cancer but it is not good to do genetic engineering to stop aging because if people they don't die there will be an increased population. In Order to reduce population if people get aged they die.