I don't know if skis necessarily become unsafe. If they've been mounted more than three times, the chance of pulling a binding out becomes significant. If they've had too many base grinds, the bases and edges become thin. And as they get skied more, they'll likely feel softer. But if they haven't had too many re-mounts, if the edges are still there, if they don't have any holes in the bases, and you like how they ski, send it!
(That being said, ski technology and shapes have advanced quite a bit recently and new skis generally ski a lot better than ones that are 10 years old)