Rule of Thumb for Older Houses

While house hunting it's important to view the house you are looking at with a paradigm or profile that will help set your expectations. Systems in houses for the most part, are designed around a 20-25 year life cycle. Some systems are designed for more like foundations and structure, other will last less, like floor boards on porches and hot water heaters. But a good general rule of thumb is the "rule of 25".
When looking at houses under 25 years old, then you are looking to see what's left of useful life of systems depending on how they were taken care of. Roofs maybe be half used up, porches may or may not need replacing or heavy repair. Window systems may be solid because they have been maintained and painted. Wood that has been painted and stained will still look good, compared to those systems that have been exposed to the weather.
With that in mind, when you look at a home that was built 25 years ago, more or less, you are looking at a house that will likely need a heating system, is on it's second or third hot water heater, needs a roof, and will likely need windows replaced. A lot depends on the preventative care and exposure from weather, some of it is just time related wear and tear.
When you look at a home that's 50 years old or so, you can expect that systems designed for a 25 year life, have either been replaced twice, or are in sore need of replacement. A heating system working that long will show its age and is way past being fully depreciated. Original roofs would be leaking and wood systems would need serious repair or replacement. There are exceptions, but this is a guide not an absolute.So if the listing on the 50 year old says new roof,new boiler, and new windows, that's a good start as it shows major systems have been replaced
When looking at houses that are 75 to 100 years old or older, you are looking at older technology, i.e., no insulation, wood shingle roofs, coal converted boilers, gravity systems, slate roofs with rusty nails, and brick with mortar failure, horse hair plaster, etc. The homes were built when the building codes either didn't exist or were being ramped up. So the support structure may be "old style" and be built with dimensional lumber, meeting minimums of years past but possibly not of today's building codes. Certain types of construction used then are not used anymore due to the design failure of those methods of construction, i.e. brick on top of rubble foundations.
Home Inspectors have this type of paradigm in their minds, tempered by experience, when they start to review houses. You can set your own expectations and be ready with questions when they review the property with you.
Bottom line, be wary as you look at older homes and keep in mind the maintenance cycles that should have gone on to keep the house up to snuff. Older homes can be more interesting and have appeal, provided that the maintenance has been kept up. So do the math before you leap!