My sites have been hit by 4 times now, I have sold it all for low prices coz of it, every time I got hit it was in September, and all sites were like 16 months to 3 years old, and it was spam update that hit me.
there is no stopping it if you have to worry about it from my experience.
Only way to stop it is good links and Good Content according google, but that's just BS, I have seen sites that have very good content and links but are not owned by huge corps got hit.
and the huge sites that copied content from these small sites took the spots for small sites, absolute bs.