To determine how old a house is, there are several common methods that can be used:

1. Check ownership documents such as deeds, titles, and home inspection reports, which may indicate the year built or previous owners' information. Local recorder’s offices can provide these records if not available at home.
2. Use online county or city assessor websites where property tax records often include the build year. Some places assign an Assessor’s Parcel Number (APN) linked to property history, including construction dates.
3. Investigate local archives, historical registries, or land registries, which may have property transfer or lease dates, old maps, architectural style analysis, and mentions in census returns to estimate the age.
4. Examine the architectural style, materials used, and design features of the house such as roofing, window style, and construction form to estimate the historical period of construction.
5. Consult local history societies, historians, or libraries for area-specific historical data or property records.
6. Use specialized online tools and calculators designed to estimate house age by inputting details about the property and construction materials.

Each method has its strengths and may be used in combination to pinpoint a house's age more accurately. For older homes, historical documents and architectural clues are vital, while newer homes often have clear records in registries or online databases. If more specific guidance is needed, such as how to find this for a particular house or location, please provide more details. This general outline can help start the age determination process for any house.