Water heaters, on average, have a lifespan of about 10 years, depending of course on how much they are used and how they have been maintained, etc. Knowing the age of a water heater is important when deciding how much money to spend on repairs and maintenance as it might make better economic sense to replace it with a new one. Here are a few tips that will help you determine whether to maintain your old water heater or replace it altogether:
One thing that can help with the decision to repair or replace the water heater is how much warranty you have left on the existing one. Even if you find it is still under warranty, check that this covers the cost of labor as well as replacement parts. Hiring someone to fix your water heater can be very expensive so if labor costs are not included, the price could really add up. By way of example, a part that frequently needs to be replaced is the sacrificial anode, this is a difficult job to do and if not done correctly can void the warranty. So, if your water heater is approaching the end of its general lifespan and this part needs replacing, perhaps you should consider whether it is worth the money to fix it or put it towards a new one.
Another way of deciding if it is better to repair your water heater or buy a new one is to calculate how much money you 
spend every year to maintain your existing one. If you have been spending more than 10 percent of the amount it would cost you to purchase and install a new one, you may want to start shopping around for a new one.
When shopping around for a new water heater, look for one that comes with a lengthy warranty as this is a sign it will be of better quality. Also, remember that you usually get what you pay for, meaning that the more money you pay, the better the product will be.