I am assuming this is your typical water-tank style water heater - the kind they sell at any big-box store.
If that's true, then they are very simple devices. Basically it consists of the tank, one or two heating elements (immersion-style heaters) and a thermostat. IMO a bad/rusted tank is the only reason you'd want to replace it (other than getting a higher efficiency model) - It is easy and cheap (~$30) to replace a heating element or thermostat if it fails.
First, check the tank, especially around the base and where the pipes go in and out. Is there rust? If so, is that rust just some surface splotches or is it pervasive (you can use steel wool or a pick to check).
Then, look at the pipes going into your tank. If they aren't leaking or damaged you are probably ok.
Finally, there's the sediment that naturally accumulates *inside* the tank. I drain mine every ~2 years. Make sure to turn the power and water OFF, and then attach a garden hose to the valve at the bottom, run it to a drain and then open it up. You'll need a faucet somewhere open to prevent a vacuum from forming. Once the tank is drained you can remove one of the heating elements (held in simply by screws) nad get a sense of whether those should be replaced and what the inside of the tank looks like.
In my experiences, unless the water tank has been sitting in a damp environment (e.g. standing water around its base or on top of a wet carpet) the tanks will last for a lot longer than 24 years. We recently replaced the heating elements in a satellite lab purchased in 1986. The tank works fine - only the elements failed.