A house/the concept of home?
I run across a landscape
Of a soft and changing terrain
Houses/homes can be built just about anywhere. Neighborhoods tend to "run across a landscape".
Rarely twice the same shape
Everyone's house is unique in some way, whether it's the overall shape or the furniture/people within.
I am unchanged by the rain
Houses are built to withstand the rain.
Some know me short,
Some know me long,
Some people bounce from home to home. Others stay in the same place for generations.
Some see me only in port
Sailors who don't consider a ship their "home" would only see their home/house in port.
For some I don't belong
By choice (i.e. nomads) or circumstance, some people don't have a home.
The older you get, the longer you live
The more likely it is to meet me
You're more likely to establish a home as you get older, often for financial reasons.
And the gift you receive I always give
The comfort given by a nice home.
A moment in time saved for memory
Homes are where most people will make a majority of their memories.
I lost faith in this answer as I was writing it, but I'm committed at this point.