Answer: -Somewhere on the nameplate it should tell what year it was made. It would be unusual to replace a heater simply because it is old; but if you have the cash for it and want to avoid a problem, then there is no reason not to. I just replaced a 15 yo heater because the T&P valve had gone bad and I figured it's time was about up. -I don't think it is normal practice, but that is exactly what I'm about to do now. My last hot water heater lasted 8 years, then one day I woke up and my basement was flooded. It's now 8 years later, and I don't want to go throught that mess again. Biggest reason for being worth it: Since lots of stuff was ruined, we called the insurance company who paid the claim. My deductible was almost as much as a water heater would cost, plus after they paid, they raised my rates, plus they only pay for the damage. You still have to pay for a new water heater. Overall, it's cheaper to replace every so many years. You decide when it's convenient to replace, instead of having to replace it within a few hours after it bursts, and dedicating the next few days to cleanup. Of course, some people's water heaters just fail, don't bust and flood a room, and last 15+ years, but with the quality of local water, it differs for each person. I will say this, and that on my street where all the houses were built within 8 months of each other, I saw a lot of AO Smith (we all had these) water heaters left for trash pickup around the same time. All our water heaters lasted about the same time.