Forever and a day, there's been an ongoing "debate" (if you can even call it that) with those who say the earth is 13 billion years old, so the Bible must be wrong and those who say that the earth is NOT 13 billion years old, and those who say that even if it is 13 billion years old, the Bible still isn't wrong.
The same type of "debates" exist concerning Noah's Ark, Jesus turning water to wine, life on other planets, etc.
So my question is, have you ever had a discussion like that with an unbeliever?
I had this discussion with myself
Or have you ever prepared yourself for such a discussion?
No, I have not prepared myself for this type of conversation, sorry.
What would you say to someone who asked you about how old the earth is or
Who cares was my first response. However after reading Gensis 1:1-2 It lead me to believe that the bible does not specifically say how old the earth is. I understand that there was some Bishop who estimated the age of the earth by looking at genealogies. However once you list the first seven "days" in the bible you get a different picture. One assumption that we make is that a day in genesis is as long as a day today. However today we know that it is possible to change the length of days. Earthquakes happen all the time, or there could be other events like meteors hitting the planet or who knows maybe the tower of Ur when it fell could have caused the earth to start to wobble (I'm joking about the wobble).
I believe this is called the Gap Theory I could be wrong.
In the end you have to take each assumption because all of these estimates use assumptions and assumptions make an

Using the fact that the length of days is not constant. It is possible to assume (remember what assumptions make though

) that the first seven days actually lasted for hundreds, or thousands or millions or billions of years.
whether there's life on other planets, and
Then there's life on other planets. If Angels can exists how come aliens can't exist. Of course the argument being presented here is that aliens prove that it is possible for a bunch of random chemicals to get together and create life. However as a christian I have not been told or read anything that said God only created things on the earth. Clearly he set about making the entire universe and it is possible for him to make some other creatures. But you have to ask him about that

how that reconciles with the Bible?
See previous answer.
Now to touch on the topic the age of the universe. It is important to realize that currently science is based on theory. And that theory changes with time and even with equipment used to measure things. For example 100 years ago we didn't know much about electrons. now we are trying to find the Higgs-Boson which is way smaller. Currently we assume that the big bang happened at a central point and the universe is currently expanding. By measuring the distance between objects you can estimate ... you know what its not even important.
Ask the person who believes this as a scientist can you recreate the big bang? All science theories you should be able to reproduce. We agree gravity works because we can drop a ball. We understand wave theory because we can watch a wave travel across a pond. We know how car engines work because we done it so many times. Can a scientist cause an explosion measure the various moleclues as they expand and predict when the explosion took place? Of course this would have to take place in a vacuum. I do dare say that they will first claim it is too complicated to accurately measure something over such a short period. Then they will complain about the various interections as the explosion takes place. Then eventually they will quit and say that they are right and that believing in something you can't prove is stupid. ... now where is that picture sjon used of the pot and kettle