The ASHI Standard of Practice for Home Inspections requires home inspectors to report on components near the end of their service lives. To figure this out, we need to determine the age of appliances such as water heaters, furnaces, air conditioners, and boilers. Once you know where to look, it’s usually quite easy to find this information. Start by bookmarking https://www.building-center.org/. I’m pretty sure this is the go-to website for every home inspector in North America. Next, find the appliance tag.
The data plate for water heaters is always right on the front. Many water heaters have the date of manufacture clearly indicated, like the one below.