The most important indicator is the average age of speakers. If you have 1000 elderly and middle age speakers but no children, the language is in serious peril.

The most important indicator is the average age of speakers. If you have 1000 elderly and middle age speakers but no children, the language is in serious peril. It is the chain from mother to daughter that needs to be maintained. Large numbers of speakers only increase the odds that that chain can be maintained.