For this they willfully forget: that by the word of God the heavens were of old... -2 Peter 3:5Now, it's true that "ancient" is a relative term. It is applied to events or people who were not millions of years ago. These verses do not prove the oldness of the universe, but they are more than compatible with the idea of the universe being extremely old. If God had wanted to, He could have in one place said that the heavens or earth were recently made or not any older than humanity, but He did not. The Bible teaches only that the heavens and earth are "everlasting" or "ancient". I find this good evidence for the antiquity of the universe.
To Him who rides on the heaven of heavens, which were of old! -Psalm 68:33
Of old You laid the foundation of the earth, And the heavens are the work of Your hands. -Psalm 102:25
With the best things of the ancient mountains, With the precious things of the everlasting hills... -Deut. 33:15
"Did you not hear long ago How I made it [various parts of the landscape], From ancient times that I formed it?" -2 Kings 19:25a
"The blessings of your father Have excelled the blessings of my ancestors, Up to the utmost bound of the everlasting hills." -Genesis 49:26a
He stood and measured the earth; He looked and startled the nations. And the everlasting mountains were scattered, The perpetual hills bowed. His ways are everlasting. -Habakkuk 3:6
His name shall endure forever; His name shall continue as long as the sun. And men shall be blessed in Him; All nations shall call Him blessed. -Psalm 72:17
His seed shall endure forever, And his throne as the sun before Me... -Psalm 89:36
Before the mountains were brought forth, Or ever You had formed the earth and the world, Even from everlasting to everlasting, You are God. -Psalm 90:2
Before the mountains were settled, Before the hills, I [Wisdom] was brought forth... -Proverbs 8:25
The above verses indicate that the hills, mountains, foundations of earth, heavens, and the sun are all old. According to current mainstream understanding of the formation of the universe and earth, all of these things are millions or billions of years old -- with the exception of some hills. Even so, most larger hills are likely still older than 1 million years old, and certainly some of them are several million years old. No matter how you look at it, these things mentioned are really old in relation to humanity, and I think that is the point.