I think it depends on perspective....... one can be very active for their age as compared to others which you might consider elderly......

for example when in high school (17 or so) I refered to a sort of hall monitor as waaaay old and he was 23...... when I turned 23.....LOL..... he was not old....now that I´m 28....... not old enough either!! LOL.......

to me an adult is between 23-30 (and all the rest of course)
an old person(not in a bad way and no meaning to offend) (like my parents age) 45-55.....(as they are not my age range and consider them other generation)
55-65 senior
and 75-??.....elderly

the 30-45 gap could be grown adult....(lacking of better term)

replace OLD with MATURE......YAY found a better term