A physician who has made a complete examination of a person without having been told his age could probably make an accurate estimate of it because of his knowledge of how the aging process works. But what would happen if he were to travel back in time and examine Adam and Eve before the fall and was then asked to guess their age? Because they were created to live forever they wouldn't have undergone the same aging process we do today. If he examined them just a few days after their creation he would give a high estimate of their ages because they would appear to be the same as adults he had examined in the past.
Suppose he went back to some time after the fall and examined them. Now they would be aging the same way we do today. If he was unaware of their past history he would make an estimate based on the assumption that they had been born in the same way as everyone else. (I know that people lived longer then so in this example I am assuming that the doctor was aware of this fact and took it into consideration. I am also assuming that he didn't notice their lack of navels, since this would have warned him that there was something different about them.) His estimate of their age would probably be off but whether it was too high or too low would depend on how much time elapsed between their creation and their fall.
Scientists who try to figure out the age of the universe are facing the same situation as the doctor in the second example. They assume that all the physical processes that they observe have been going on since the universe began. But the Bible tells us that this isn't the case. God cursed the ground because of Adam's sin. But it appears that the whole universe is under a curse, which will be lifted at the same time as the resurrection of believers. "The creation itself will be set free from its bondage to decay and obtain the freedom of the glory of the children of God." (Romans 8:21) It seems that the whole universe was affected by Satan's rebellion and that is the reason for the condition it is in today. Since we don't know how much time elapsed between the creation and the fall of Satan it is impossible to figure out the age of the universe by observing what is going on in it today.
Here are two sites that present alternative views to the generally accepted theories about origins:
Answers in Genesis - Creation, Evolution, Christian Apologetics
Science Against Evolution Official Home Page