The word senile describes a person who is experiencing dementia brought about by old age — in other words, someone showing signs of senility.
Senile can refer to a loss of physical abilities in old age, but it usually refers to decreased memory and mental faculties. Getting senile is a very serious condition, as an older person might have trouble remembering who their own children are. Many people in nursing homes are senile, because their families aren't able to provide proper care. Of course, people forget things at every age. If you're 10 or 40 and have a bad memory, you're not going senile.