Aging begins when you are born IMO, or whatever point a person comes into existence.

I don't think creatures who reproduce ought to be immortal.

Senescence, I agree with the people who put it around 25, the point where you stop growing at all physically has to be the point you start degrading, but it must happen so much slower than growing does, I felt my absolute physical best around 45 years of age (certainly NOT 25) and still feel really good at near 60. So maybe there was an adjustment period then a steady state for awhile. Or a quick slide then s slower slide?