Our house was built in 2003 and it never occurred to me to pre-emptively replace the hot water heaters. That said, we have an unfinished basement, and the heater is near a drain, so I'm not too concerned.
Horror story for you: our old house was on the market, and had a showing. They happened to notice that there was a small leak with the water heater. The house had a fully finished basement.
We called the gas company (gas hot water heater) and said we were told the water heater was leaking, what should we do? They said "if it COULD BE a gas leak WINK WINK NOD NOD we will send someone out for free to check it". So they send someone out, yes it's leaking slowly, no harm done yet. They shut off the water, we called a lumber and got it replaced with no damage. It could have been a huge major disaster, but thankfully was caut just in time. The heater was 18 years old. If I ever have one in a finished basement again, I will certainly be more cautious.