they are offering up "the prayers of the saints" who are
on the earth (Rev 5:8-10, cf. Rev 8:3-5).
This appearing can't happen before the tribulation
because it destroys the Antichrist:
2 Thes 2:8
Then shall that Wicked be revealed, whom the Lord shall
consume with the spirit of his mouth, and shall destroy
with the brightness [epiphaneia] of his coming.
The 24 elders have received their crowns already:
Rev 4:10-11
The four and twenty elders fall down before him that sat
on the throne, and worship him that liveth for ever and
ever, and cast their crowns before the throne, saying,
Thou art worthy, O Lord, to receive glory and honour and
power: for thou hast created all things, and for thy
pleasure they are and were created.
We won't receive our crowns until Jesus appears,
after the tribulation:
Matt 24:29-31
After the tribulation... Then shall appear the sign of
the Son of man in heaven... and they shall see the Son of
man coming...
Rev 2:10
Do not fear any of those things which you are about to
suffer. Indeed, the devil is about to throw some of you
into prison, that you may be tested, and you will have
tribulation ten days. Be faithful until death, and I will
give you the crown of life.
The scriptures don't say who the elders are. They
can't be the 12 sons of Jacob and the 12 Apostles,
because John the Apostle saw all 24 of them.
Peter and John were able to recognize that it was Moses
and Elijah at the transfiguration, but John doesn't mention
who any of the elders are. They’re probably as ancient as
the 4 beasts.
They and the 4 beasts are singing not for themselves, but
they are offering up "the prayers of the saints" who are
on the earth (Rev 5:8-10, cf. Rev 8:3-5).