I mentally chop adult age groups at 18, 21, 25, 40, 65, and 90. Within those groups, I don't really conceptualize any difference. Two clarifications: (1) this is talking about a single person, not a group, and (2) it's really more based on behavior than age; my grandma is in her eighties, but is so mentally sharp that I think of her in (the top end of) the 40-65 group.