Nowadays, social protocols regarding forms of address in 'polite society' in western English-speaking countries are much more relaxed than they were 100 or 200 years ago, especially in the USA (and even in stuffy old Britain).
Unless you're part of the Hollywood crowd or a London West End actor, most people would probably reserve 'Darling' for people with whom they have a romantic relationship. 'Dear' also implies a certain degree of (at least emotional) intimacy.
But I don't think age differences are much of a factor now in terms of the general expectations that people have about how they ought to address one another.
The chief exception is perhaps such limited circumstances as where a large status difference is coupled with an occasion of some formality, especially when it also involves an official hierarchy with its associated rules of proper conduct (such as pertains to a branch of the armed forces).