As dragons grow older, they become more resistant to spells and spell-like abilities, as indicated in the dragon descriptions. A dragon’s SR is equal to 11 + its CR.
But many of the younger dragons are not depicted as having SR in their individual entries. Which is controlling?
The special abilities table for each Dragon type in the Bestiary lists "spell resistance" at Young Adult, so that's the earliest any of them gains SR naturally. In theory, other Dragon variations could get SR earlier or later.
That's what "as indicated in the dragon descriptions" means. Before the Dragon is old enough, it doesn't have any SR at all. Once it gains SR, its SR is equal to 11 + CR.