How old was Adam when God created him?
There are a couple technicalities I'd like to get out of the way before I attempt to answer this question directly.
First, we should be aware that there are some Christians who believe in theistic evolution. Such Christians believe there is scientific support for the idea that humanity evolved from lower life forms over millions of years, but they insist that this process was guided by God. If this view is correct, then "Adam" was just part of that evolutionary line and was not specially created.
For the record, I do not support the theistic evolution viewpoint. I believe that Adam was specially created by God on a specific day.
Second, I'd point out that technically (in the special creation view) Adam was 0 when he was created by God. If our age represents the amount of time since we came into existence, then Adam was 0! But I suspect the question really being asked is something like this: How old did Adam appear to be on the day he was created?
Obviously, we can't know, for sure, the answer to that question. The Bible certainly doesn't tell us how old he appeared. The only numbers we know are that Adam was 130 years old when Eve gave birth to Seth and Adam was 930 years old when he died (why these numbers are so large could be the subject of a future Q&A for Today!).
So we are left to speculate as to how old Adam would have appeared to be on the day God created him. Certainly God, in divine wisdom, would have created many things in a mature state (plants, animals, starlight, etc.).
Given the fact that God immediately called Adam & Eve to be fruitful, I think we may safely assume that they were created in a post-pubescent state. And I can't think of a good reason for God to have created them to "appear" much older than that.
I'd guess that they were created with the physical appearances of post-pubescent young adults. Some people speculate that they were created at the peak/prime human age, but I'm not sure exactly what that age is (late 20's, early 30's?). We do know that Jesus reversed the curse as the "second Adam" when he was "about 30 years old" (Luke 3:23). I'd guess, then, that Adam was somewhere between 13 and about 30 in terms of physical maturity.