The oldest tank I know of that's still up and running, was built by O'Dell, which used to be here in Canton, back in 1973. It's in remarkably good condition for being nearly 41 years old - but it does have a bit of wear and tear on it, but it still holds water just fine (knock on wood).
I sold a tank a few months before I closed, that I bought new in 1988. I'd taken it down a while before that but it held water when I took it down and I had no reservations about it.
Much depends on how it is cared for - if the silicone is abused, scraped, etc., it probably won't last as long, but if care is taken not to damage the silicone, and it's on a good stand and level, it should last for many, many years.
Jenn