Your water heater is, indeed, probably past it's design life, but that doesn't necessarily mean that it's going to burst at any moment. I've seen these suckers go for 40 years. And while bursting is possible, it's very rare. In most cases, they dribble for some time. If there's a drain pan under the water heater, you might consider just waiting until it starts to leak. Of course, it'll probably happen at the worst possible time (the day before Christmas?) and you'll be forced to make the replacement quickly, without having time to plan for the repair. That said, whether or not you decide to replace the water heater proactively is up to you. Make the decision based on your own personal tolerance for risk. If the system has been working well for you up until now, then when you replace it, be sure to replace it with another heater that has the same capacity in terms of gallons and btu/hr (or kw, if it's electric). Going with a smaller tank wouldn't provide any economy. When you decided to replace it, shop around to find a tech that you trust and that you feel communicates well. As usual, avoid the low bidder. Thanks! I feel a lot better about this. For now until I see an actual problem, I'm going to wait a while before replacing the water heater; however, I am going to write up a contingency emergency plan, just in case.
