What it looks like on the outside says nothing about what’s happening on the inside. At 15 years, it might be OK, but it might well be on its last legs, especially if it was a 6-year-warranty tank.
If you go to The Basics section, using one of the links on the left side of the Tank topics index, and click on Know-How, you can find external inspections to do for both electric and gas heaters. If those look OK, you could pull the anode and check it.
But I have a feeling that no matter what you do, they’re still going to demand a new heater.