aged adjective 1 | ājd | [predicative or postpositive] having lived for a specified length of time; of a specified age: young people aged 14 to 18 | he died aged 60. • (of a horse or farm animal) over a certain defined age of maturity, typically 6 to 12 years for horses, 3 or 4 years for cattle. 2 | ˈājəd | having lived or existed for a long time; old
"Old" is one of the definitions of aged.
"Aged" is a very marked word in contemporary English. And "senilis" is pretty marked in Latin, too. Please note that the PHI doesn't give a single example of senilis referring to a person (Servius uses it once of animals). Pater eorum est senex is how to express this idea in Latin.
Yes. It's genitive plural of "is," which translate to "he."
This link has the declension tables: