I work at Krown right now, and I tell you, I see older cars come in everyday for their annual coating.
One i'm never gonna forget was about a week ago, a 1994 Chevy 2500 ext cab with 490km (306k miles) had been Krowned every single year since new, it was still under Krown guarantee! Not a single spot of rust on that thing!
That's the thing with undercoating, you have to do it right away when the vehicle is new. If you decide undecoating over rust... it's pretty pointless. It will extend it's life abit, but not enough to be worth it. But if you do it straight from the start, every year, it really does help!