When to replace a modem [obsolete]?

[kiddingguy]

I have a question... when is it wise to replace a modem because of possible security issues.

In some cases I know people who have their modems on firmware from 2014 and earlier, with no official FW-updates from e.g. Linksys, Asus, etc.

 

Are these hackable? Either from a distance or via de drive-by?

And what possible damage can cause this for the specific person in question? (like being part of a botnet of spamming)