How do you know when it's time to replace a fish tank?

[Kit with Cat]

Hey guys,
Just wondering if there's a rule of thumb as to when it is a good time to replace a fish tank. I bought mine second hand, and have been using it for almost 5 years. The silicone adhesive is starting to peel when I scrape the glass near the corners of the tank, though that's about it.
Any help would be much appreciated. Thanks!