Also, to add to Artak's description.
You know how you can spot an enemy vehicle or unit?
And it shows up bright red on the map?
Well, that is info age 0.
You know exactly where he is right now, and it's bright red on the map.
But if you lose sight of him, and one minute passes, the info age will be 1 minute old, and the bright red marker will have faded a little bit.
The higher the info age, the more faded the marker, the more uncertain the information.
edit
Practical applications can be if you simulate a recon-flyby or something in the beginning of the mission, telling the player where the enemy tanks are.
But from the recon to the actual start of the mission 5-10 minutes passes, yeah?
So you set the info age on those units to 5-10 minutes.