What are earlest signs of senility?

MCI: Mild cognitive impairment is the first sign of dementia. This means trouble remembering things you used to, or trouble managing things you used to be able to do (e.g. Planning thanksgiving dinner). It isn't very specific-- illness or stress can also cause the same problems-- but mild cognitive impairment can lead to dementia including alzheimer's disease. Consider a checkup for early treatment.

What do you mean?: Senility is no longer used for any problem an elder might have. If it is memory problem, it may be alzheimer's disease but it could be also a reversible cause that requires medical examination.