Think about all the different documents that list an age for your relative. Can you use those to reach any consensus about when the person was born?
It is not easy, but can be done. Here is a link to several ages for an ancestor of mine and a chart I made to analyze his ages. Not the most professionally done chart, but it serves the purpose.
We'll be devoting a whole issue of Casefile Clues in the future to working with ages, but this should be enough to get "Tip of the Day" readers started.