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 a young age in his or her life (one notable exception is Amenhotep IV).
If God inspired any of the above depictions we might surmise that the first man, 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 thirty.
When Jesus was killed on the cross he was in the prime of his life at age thirty-three and one half. Interestingly, the Bible refers to Jesus as the Second (or last) Adam (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 twenty-five and thirty-five years old. This lends some evidence to the argument that the first man was created in this age bracket.
Did Adam fully comprehend the world around him at the age he was created to be and was emotionally mature enough to handle what came at him? The Bible does not state whether he was or was not. If he had lived longer in the world of wickedness, however, he would have seen the wisdom of doing things the right way. Instead, he sinned and chose to decide for himself what was right and wrong, instead of trusting in his Maker.
One indication that Adam was emotionally like an adolescent or younger child occurred when his sin of eating the forbidden fruit was discovered. Rather than owning up to it like a mature person and asking for forgiveness, he chose to lay the blame on another person (Eve) and hide himself. She, of course, followed his example and laid her sins on the serpent (Satan) who beguiled or deceived her.
In conclusion, it would be safe to say that Adam was likely created by God to look, physically, like a human being in the 25 to 35 year old age range.