When cared for properly, semi trucks can last for a long, long time, but sometimes that age isn't an advantage. Due to changes in laws and technology, older trucks are often not suitable for driving in certain conditions -- including locations like entire states. While this doesn't mean you need to buy a completely new truck, you would do well to buy new-to-you trucks on occasion. If you do, you could find that your ability to work for different companies and travel through different states might improve drastically, which will also improve your chances of earning more money.
Much of the restriction placed on trucks due to model year has to do with emissions. For example, the California Air Resources Board (CARB) requires any drayage trucks in the state to have engines that are from at least the year 2007; by 2023, the engines must be from at least 2010. So if you would like to drive and transport goods in California and you're planning to be part of a drayage fleet, your truck needs to be at least from 2007 if not a more recent year. (It's not unusual for trucking companies who contract with owner-operators to set their own requirements.)
Another issue is repair frequency. Older trucks that are in good shape will still run up against limits on what parts can do. Maybe you've done your best to keep your old truck running, but if you're approaching that age for the truck when lots of parts tend to wear out and the engine needs to be rebuilt, you may be better off buying a used Freightliner with fewer miles. Obviously, that depends on the final costs and the condition of your truck overall, but it is something to consider. If you're constantly taking the truck in for repairs, that limits your time on the road and thus limits your earnings.
Older trucks may not be as fuel efficient as newer trucks. That's just a result of technology improving over the years. It's like having an old, trusty fridge that never breaks down but that uses so much energy it's become too expensive to hang onto -- that's definitely a sign to get a newer one. The same goes for trucks. If you need to conserve fuel costs and save money, that would be a benefit of buying a more recent model. In the short term you'd spend a lot more, obviously, but if you're going to drive that truck for years, calculate the costs -- it could work out in the more recent model's favor. With better mileage, you can go farther and be less worried about accessing fuel pumps.
You aren't obligated to buy anything, so start looking around at used Freightliner truck dealers. You may find a financing program and a model that make the transition worthwhile. Check out a website like http://www.arrowtruck.com/ for more information and assistance.