I'd like to say 100 years is old- a house from last turn of the century is old. But just a year span isn't really the whole definition for me. I often call a house old if it was built without indoor plumbing or electric- like an urban house from the 30's can be old, or some more rural homes into the 40's can be old. Houses built without ventilation in the walls for heating or cooling usually gets called old by me too, lol.

Older homes for me usually run in between the 40's-80'sish. Has built in plumbing, electric, and ventilation in the walls. Modern kicks in late 80's-2010ish. Has plumbing, electric, ventilation in the walls. Usually has "model home" or "McMansion" sorts of vibes with built in kitchens and the start of using stone for counters.

New or newer is around 10 years old- though that can vary a bit too by a few years. If it still has it's original parts and working good, that's newer. If stuff like roofs and flooring needs replacing already, that is not so new anymore.

I think of old from two aspects: romantic or commercial.

Around here, I'd say a house is old if it was built in the 1930s or earlier. Anything built after that shows a big change in style, construction type, materials, etc.

I think that it has to do with the age of the person rather than the age of a house. Someone posted the other day that a 30 year old house was too old to be considered or something like that. A lot also has to do with the design of the house although some very modern looking homes were built in the Bauhaus school style that would for some look totally contemporary.

100 years is old.

My definition is a bit older than I am, about 1960 or earlier counts as old.

We are buying a house where the “modern” addition dates from 1829, with the “old” part of the house dated from the mid to late 1700’s, so old is really relative depending upon the setting, although I’d tend to agree with 100+ years.

Wide variation in "old house"

https://www.thisoldhouse.com/project-houses

Season 37 - The Belmont Victorian House

This Old House renovates a three-story 1895 Queen Anne in the Boston suburb of Belmont.

Season 40 - The Brookline Mid-century Modern House

This Old House modernizes a mid-century home in the suburbs of Boston.