ANSWER: You have asked a good question about Adam that unfortunately is not definitively answered in the Bible. That said, we can draw a general age range from inferences made in scriptures and from man's history.
When Michelangelo sculpted the statue called Slaves in the 1500s he depicted his subjects as young men in their prime at about the age of 30 or so. He also painted Adam in his prime when depicting him on the ceiling of the Sistine Chapel.
Archaeologists that have uncovered tombs and mummified remains from the ancient Egyptians have found that most of the statues and masks discovered depict a person at an age when they were in the prime of his or her life (one notable exception is Amenhotep IV). Was this the pride of man to want to look his best for posterity or was this the inspiration of God to do these art pieces in this manner? If it was his inspiration then we might surmise that Adam, who was sculpted out of the red earth of the Middle East, was created in the prime of human life. This prime would be roughly around the age of 30.
When Jesus was killed on the cross he was also in the prime of his life or half way through his earthly lifetime. He is called the Second (or last) Adam in the Bible (1Corinthians 15:45).
We know that after our first parents left the Garden of Eden they were young enough to procreate many, many children. When we study all of the "begets" found in Genesis and elsewhere we see that the first child in each family is usually born when one or both parents are between the ages of 25 and 35. This lends some evidence to the argument that the first man was created in this age bracket.
How long Adam and Eve were in the Garden of Eden before they sinned is not known. We know that their lives were not subject to death until they sinned. Their life span was counted from that point forward. Did God teach them his laws and educate them in the disciplines he thought were necessary for their understanding? He gave Adam the sole task of naming all of the animals. One would need to know a little bit about biology to be able to do that I would think. This also would take a considerable amount of time. God also gave them the dominion over all of the earth with the express command to dress and keep it. This would, of necessity, require some knowledge of the physical world around them.
In conclusion, it would be safe to say that Adam was likely physically created by God to look like a human being in the 25 to 35 year old age range.