First, someplace on the water heater it should state how long the warranty is or when the warranty expires.
If the warranty period is 5 years, you are probably out-of-warraty. If a 10 year warranty, you are probably in-warranty.
That depends on the following: the typical way to read a serial number (with a number like yours) for the date of manufacturer is like this is – the first four digits are the week and year or year and week. Thus your serial number, Serial#089125045, would be the 8th week of 1991.
Different brands use different codes, however, your s/n date code fits that format.