I'm not sure how to explain the question, so I'll do it by an example.
The median age in some country is 41.7. 50% is younger than 41.7, 50% is older.
How would I calculate the percentage of the population being younger and older than say, 31.4, in that country, based on 41.7 being the median age?
Even if you know the median, you would need to know what distribution it is (the shape of a graph of the data) because it could be skewed to one side. For things like age, it's likely to be a normal distribution. If that is the case, then you'd need to know more about the data (like the standard deviation or variance or total number of people etc.) to help solve your problem. In other words, knowing the median could tell you where the middle of the distribution is but you don't know how much it's stretched out sideways.