The use of obasan/ojisan and obaasan/ojiisan in Japan with reference to age whether accurate or meant to insult is pretty loose and I don't think there's a written rule of what age equals what word.
I use obasan/ojisan for middle aged people, but never to their face (I could be wrong, or they have a personal preference).
I use obaasan/ojiisan in reference to fairly elderly people with wrinkled faces, snow white hair (if any). While you might see someone address a perfect stranger as obaasan (like in hospitals), I would just use caution and fair judgment that the person you're talking to is without a doubt worthy of that word.
Here's a list of more words that refer/address people not based necessarily on age, some context may be necessary - use at your own risk:
ojouchan - little miss, milady (female, can also end with chan/sama)
bocchan - little master, young sir (male, also can use chama)
oyaji - pops (male, in media they use this to address strangers as well as familiars)
ossan - pops (male, less familiar, can be rude if used for stranger)
niichan - bro (male, used between familiars and strangers, can also use san)
neechan - sis (female, impolite, similar to niichan)
baba - grandma (can be used normally depending on family, can also mean "old hag/crone")
jiji - gramps, grandad (similar to baba, can also mean "old timer", "old fart")
boku, bouya or bouzu - little boy (used when addressing small boys)
kozou - little boy (but closer to "little runt", "little bastard", "little rascal")
dannasan - sir (can also mean husband)
okusan - madam (can also mean wife/missus)