For all age groups.
This question makes sense if you understand God to have a beginning just like other things and beings. Indeed this world is full of "gods" who once had their beginning when man invented them. This kind of "god" however is not what we are talking about here:
The question above is based on a misconception. It is true that all things which had a beginning must have had a cause as well. At the same time it is absolutely not true that everything which exists, had a beginning and allows us to ask for the cause of that beginning. Otherwise, how did the first thing with a beginning come into existence? Without an eternal SOMETHING - there would be absolutely NOTHING now.
But what is this eternal something? Well, even if you are not into religion, you do believe in eternal concepts, don't you? Have you ever asked yourself since when 1 plus 1 equals 2 or who invented the laws of logic and mathematics? Such laws have been and are valid at all times, even when no one knew about them. Logic is an eternal concept, despite the fact that people often think and act illogically.
These eternal concepts cannot be touched nor seen directly. We may have developed symbols but still: Even if no one knew how to write numbers, they would exist unconfined by time. Likewise God's existence does not depend on how many people believe in Him. His eternal being, His character, are not subject to change.
But God has to be more than an abstract law, more than an idea like Greek philosophers thought. Mere knowledge about the addition of numbers won't put the corresponding amount of Euros into our pockets. Laws of logic alone don't cause practical action. We and the world around us would still not exist. But we ARE here and you can read these lines.
The Bible teaches that this eternally existent God spoke and called into being everything which had not been there before. (Psalm 90:2) We are left with the choice whether to keep on inventing our own gods or to start seeking the One who gave us this life. Jesus promised the honest seeker that God will let Himself be found (Matthew 7:7-8).