If there is a central heat and air unit (around here they're gas or electric, so I can't give any information on oil or steam units) you can find out how old the unit is by checking the information on the sticker if it is readable. I was told that the date of manufacture will be part of the serial number, but I have found it in various places on the tag, so if nothing looks like a month and date in the serial number, check the model number if it's not there either, keep looking. The date of manufacture is printed on the sticker somewhere. On our old York units, the mfg date is in a separate area of the sticker.
As you can see in the area circled in red, this particular unit was manufactured in August of 91. While it works, it isn't efficient and is experiencing some problems that cannot be corrected. It's well past its life expectancy and will need to be replaced.
This one is newer-June 96, but is also failing as the air function is completely out of service. The life expectancy of a unit is about 12 years. At the 12th year, the air went out. Annual maintenance also comes into play when extending the life expectancy of a unit. A unit should be cleaned, inspected, and serviced in the spring before the summer season and again in the fall before winter. Neglecting the annual maintenance tends to cost more in the long run as the unit will need to be replaced sooner. It's a good habit to get into. A quality well-maintained unit can last up to 20 years or more.