I love it when various books on the Tribulation have them as weather beaten tough men of indeterminate age. I see them as older but not ancient. Muscular, forceful men ready to blast their enemies with fire, able to shout over a crowd and make everyone hear them whether they want to listen or not.
Here's a good description of Moses at his death, and he doesn't sound old at all. He climbs a mountain, his eyes were clear, his strength was THERE! I've seen videos of teachers climbing that mountain and it's a pretty steep gruelling climb for younger men.
Deut 34:1-
1 Then Moses climbed Mount Nebo from the plains of Moab to the top of Pisgah, across from Jericho. There the Lord showed him the whole land—from Gilead to Dan,
2 all of Naphtali, the territory of Ephraim and Manasseh, all the land of Judah as far as the Mediterranean Sea,
3 the Negev and the whole region from the Valley of Jericho, the City of Palms, as far as Zoar.
4 Then the Lord said to him, “This is the land I promised on oath to Abraham, Isaac and Jacob when I said, ‘I will give it to your descendants.’ I have let you see it with your eyes, but you will not cross over into it.”
5 And Moses the servant of the Lord died there in Moab, as the Lord had said.
6 He buried him in Moab, in the valley opposite Beth Peor, but to this day no one knows where his grave is.
7 Moses was a hundred and twenty years old when he died, yet his eyes were not weak nor his strength gone.
Elijah's last physical description is in 2 kings 1 here:
7 And he said unto them, What manner of man was he which came up to meet you, and told you these words?
8 And they answered him, He was an hairy man, and girt with a girdle of leather about his loins. And he said, It is Elijah the Tishbite.
The description of him going up in the chariot is the next chapter but my mental picture of the both of them is a blend of both these passages.
And of course there is the Transfiguration of Jesus at which Moses and Elijah appear described here in Luke9: 30-31
30 Two men, Moses and Elijah, appeared in glorious splendor, talking with Jesus.
31 They spoke about his departure, which he was about to bring to fulfillment at Jerusalem.
Which sounds like they both appear with Jesus also in a glorified form.