When does a historical marker become historic?

This one was erected 90 years ago by a long defunct railroad near Grenville, New Mexico. It reads:

First Wagons used on Santa-Fe Trail crossed here in 1822 

Erected by Colorado & Southern Railway 

J.D. Walker, Supt. - Robt. Rice, V.P. & G.M. 1930 

It's already been damaged by vandals, so maybe it would be better placed at the Museum in Clayton?