The transmission in your vehicle is among the most important parts of the entire vehicle. Are you going to trust it to hold up or are you going to take the steps you need to if it starts to show signs of problems? You should take corrective action as soon as you realize that there could be an issue.
A transmission warning light will be your first clue, but there could be more than that. If the vehicle is leaking fluids or if it is having trouble moving into gear, then there could be a major issue.
The transmission is only built to last so long, so make sure you are prepared to take care of it in the event that it starts to slip away. It is pretty easy for this to happen, and you don't want to be caught unaware when it does. You have the power in your hands to get it taken care of.