I've never done hot-air soldering but am helping set up a lab. I
understand we need solder paste and its shelf life is short. But I
just found some solder paste that has been out of refrigeration more
than 7 years, and it still looks like paste. What quality has it lost?
Stickiness?

How important are reliability and traceability (ISO 9000) in your lab?

So can you tell me what happens to solder paste when it gets old?

No, that would be your problem.

Precisely -- I asked the question because I wanted to know the answer.

Can anyone tell me what happens, physically, to solder paste that gets old,
and why it's no longer considered suitable for use? I know the
specifications say not to use it.

More generally, if I didn't know whether some solder paste was fresh, what
changes should I look for?