Good question.
A trailing dog will follow the "scent" of a person that is cast of their body (which is made up of skin cells (think micro dandruff) that are being shed off, gases, oils and a whole multitude of other things) These micro particles leave a "trail" as a person walks. When a person stops, they "pool" or become thicker/more concentrated. When a person runs, they become more dispersed.
So, a fresh scent means a trail that has recently been left. In other words it is from someone who has recently walked/ran/crawled/etc that path, minutes to hours old.
An aged or older trail is going to be one that has been there for a day, to days, to weeks.
Scent does different things depending on how long it has been there. When a scent trail is "fresh" it is still up in the air, it is in a tighter "trail" because it hasn't had time to settle or be affected by the elements. As time goes by the scent disperses a bit, making a wider but thinner trail, it pools and hangs around vegetation, curbs, barriers.
Weather plays a huge part too, hot, cold, damp, dry, torrential rains, winds.... all are going to impact a trail.
Does that help?
Temperature,