Some websites (including this one) don't function with a Safari version as old as the one incorporated in High Sierra. If you need more detail than that you will need to discuss what you don't want to discuss. Keep using it until it no longer serves whatever purposes you bought it to serve.
It's not age as such; it's the fact hardware is unable to change, grow, adapt or evolve to accommodate evolving industry standards.
I have Macs even older than yours and they still work as well as they did when they were new, but some of them predate the iPhone, Twitter, YouTube... etc. They have become little more than curios; dust collectors that belong in a museum.
Designing a product for things that have yet to be imagined is challenging, but Apple excels at it.