Why do some Christians say the Universe is billions of years old, but mankind only six thousand years?

Your question needs more details.

What do you mean with why?

There is no age given by the Bible in regard to the age of the universe except that it is very very old. There are two scriptures, one simply says 'in the beginning' and that begs the answer to when that beginning was, and for this we have this: ' whose goings forth are from of old, from everlasting.'

This phrasing 'from everlasting' occurs in two different scriptures, and surely permits us to say that the beginning must have been very very long time ago, perhaps millions or more years ago so that a human that lives 60-80 years would find it more or less 'from everlasting.'

Scripture does indicate that Adam was created ca 6000 years ago.

There you have it.

I have not met one that believed that, so your experience might be fairly unique. Christians who believe humanity is only 6,000 years old, are a small minority of perhaps 3%. But they almost always believe the earth and heavens were created about the same time, in a literal 6 day period. Some Christians might believe that God shaped the universe rather than creating it, but they tend not to believe humanity is only 6,000 years old, also. There are all kinds of combinations of beliefs among Christians, leading some, especially Catholics, to say that every individual Protestant is his own denomination. There is a wild variety of beliefs among Catholics as well.

“The ancient allegories in Genesis 1 - 11 were never meant to be read as exact histories, and certainly never as science texts. This has been well documented by respected theologians such as Augustine and Thomas Aquinas. The overwhelming majority of Christians (and Jews) know that the earliest books of Genesis were handed down from generation to generation by oral tradition, before the Hebrew people developed a system of Phoenician writing, around 1000 BCE.

The American fundamentalist movement, is a fairly recent development, and goes against all true Biblical scholarship. Those who profess to take the Old Testament "literally" without even bothering to research the Exegesis and Hermeneutics only publicly display their ignorance, and manage to make all Christians appear delusional in the process.” Mariel

There are things which they can no longer deny, like the age of the earth and the existence of fossils proving that many more species of animals have become extinct than are around today. So they have to adapt their beliefs, and in my view compromise them, to accommodate what they know to be factual. They continue to somehow fit the Biblical stories of universal floods and Genesis into the known facts. Some have even gone so far as to accept that god is behind the Big Bang. How that is compatible with Genesis beats the hell out of me!

Because the ideology of fundamentalism requires that anywhere a wooden reading of the Bible conflicts with empirical evidence, no matter how compelling, it is the evidence, not the Bible, which must be questioned. Different fundamentalists draw different lines at which to reconcile their wooden Bible-reading with the evidence of reality. Thus we get weird hybrid theories that sneak billions of years of universe-expansion in, before Bishop Ussher's wooden chronology from Adam.

Some Christians accept modern science.

Some Christians aren't arrogant enough to believe that the entire universe was created to give humans something pretty to look at.

PERHAPS 6000 YEARS AGO THAT IS WHEN MAN EVOLVED A 'COLLECTIVE CONSCIENCE'.

That sort of aligns with my beliefs that when God and Jesus were saying that now man has become like them , knowing good from evil ; For , archeology points out that since man was burying his dead nearly 100,000 years ago THEN it must mean that they had evolved a "conscience" . But , in line with the story about Cain and Able , maybe Able was a hunter-gatherer who had not evolved a "collective conscience" yet . And , maybe he was always stealing from Cain , who had evolved a "collective conscience" ; And Cain got tired of it and killed him . So , Cain had the ability to feel guilt because he had a "love-for-others" conscience , but Able didn't . But Able faired better because he was able to enter the "Happy Hunting Ground" , whereas Cain had learned "GUILT" now , so would have to seek and find forgiveness before he could ever find REST in the after-life .

I've never heard a Christian say that. YEC think the universe is young too.

The traditions helped along by a man named Bishop Ussher.

http://en.wikipedia.org/wiki/Ussher_chronology

Me don't likey traditions like that.

I'd say that most havent' even heard his name mentioned before.

They take the Biblical word for "Day" used in Genesis as being ambiguous and possibly meaning BILLIONS of years, instead of traditional days...