Where does it say the 24 elders are human?
How about the saints under the alter?
What, then, are they? They are not angels. If kerryusama's contention is correct that there are only three humans in Heaven mentioned by Scripture, then what is left to explain the presence of these elders?
In every single case in the Bible where the word "elders" is used, it refers to human beings. What else could it be here? Besides, Revelation 7:13-15 not only mentions one of these elders again, it also has him asking St. John a question: "Who are these clothed in white robes, and whence have they come?" John relpies: "Sir, you know." And then the elder responds: These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb. Therefore are they before the throne of God, and serve Him night and day within His temple..."
Please, what could be plainer? Not only is the elder spoken to by St. John clearly a human, but he makes direct mention of a multitude of martyrs (definitely not angels!) who "are" before the throne of God (i.e.: in Heaven), *before* the end of the world, as the seventh seal hasn't even been opened yet. Check out also, if you please, the reference to this host of people in Rev. 7:9-10, immediately preceding John's conversation.
It is simply ridiculous to suppose that Heaven will be uninhabited by the souls of humans before the End.