I expect this is universal, but the city I live in has suburbs of different ages like growth rings of a tree. The earliest ones are in what is now considered the core; further out ones have aged to sometimes have interesting character of their own, all the way out to the ever-expanding exurban sprawl.
I simply live in a "growth ring" in which I feel comfortable. My house was built in 1970. There is no "urban / suburban" dividing line. It's a matter of perspective.
This is a really interesting way to look at it. This weekend we were driving through the Sunset in San Francisco, and my daughter asked if it was a suburb of San Francisco. We told her technically no, because it is within the city limits. But on the other hand, most of the area was built between 1920 and 1945, which was at least 75 years after most of the downtown core (before the earthquake). So in that regard, it's definitely an early "growth-ring" of San Francisco.
Suburbs and cities grow in so many different ways; some explicitly, some implicitly.
Around where I am, the "town" expands to encompass new developments; but some are almost suburbs (of an exurb) - other places the suburbs grow as independent entities until they're pushed up against each other.
The failure of the postwar subdivisions was, paradoxically, a result of their great commercial success. The making of suburbs, which had been an honorable branch of town planning, became simply a way of marketing individual houses. By concentrating entirely on making houses affordable, the developers overlooked the chief lesson of the 1920s garden suburbs: subdivisions should consist not only of private dwellings but also of public spaces where citizens can feel that they are part of a larger community.
Today's formula for unlivable areas and car dependency was already old-hat in the 1930s: sleeping regulators who allow profit-seeking development with no regard for livability generate inter-generational pain points for society as a whole.
Seems to me that things are coming full circle, with people increasingly living in higher density apartments and the better apartment complexes providing gardens and social spaces by private-proxy to public planning.
A friend has a standalone house that’s 2.5 stories. He lives in the outskirts of Hiroshima Japan. From his house it is a five minute walk to two grocery stores. Fifteen minute walk to the train station. A bike ride to the local school and to the ocean. I am jealous. The lot his house sits on is tiny though; about a third of my house in the US. The streets are narrow with no sidewalks. His wife drives to perform errands. Everyone else walks bicycles or takes the train.
I simply live in a "growth ring" in which I feel comfortable. My house was built in 1970. There is no "urban / suburban" dividing line. It's a matter of perspective.