Was Hades the eldest of the gods?

Answer 1

Yes, and no.

The Titans Cronus and Rhea had six children, while Hades was born first he was quickly digested by his father, as well as the four children after him, finally child number six (Zeus) was born and Rhea tricked Cronus into thinking he had swallowed him too. This was not the case, Zeus was in fact the only child to grow to maturity outside of Cronus's body, which eventually allowed him to fight Cronus and free his siblings.

So in summary, an appropriate statement would be "Hades was the first of the six original Olympic gods born to the Titans Cronus and Rhea."