In every case I'd first try to contact the missing adults by their cell phone. Then I'd contact friends, neighbours and family members. If neither has seen the missing person I'd report them missing:
1) an elderly, frail relative who doesn't live with you: after 1-2 days (One must allow them time to get into contact. They might be in hospital or staying with friends)
2) an elderly but active relative who doesn't live with you: after 3-4 days (they might be on a spontaneous short trip)
3) an elderly person who lives with you: if he's not at home for supper (6 pm)
4) an adult relative (excluding elderly) who doesn't live with you: after 2 weeks (he might be on a vacation)
5) an adult relative who lives with you: the next morning.
6) a spouse: if he's not at home for supper (or if he's more than 3 hours overdue for an appointment)
7) an adult child who doesn't live with you: after a day or 2 (might be visiting friends)
8) an adult child who lives with you: the next morning
9) a teenager: the next morning
10) a pre-teen (10 to 12 year old): if he's not at home by 8 pm the latest
11) child (6-9): if he's not at home by 7 pm
12) child under 6 years of age: A 6 year old if he's not at home for supper (6 pm); a child under 6 I'd report missing immediately as they have no business to be off the premises without asking me first.
13) a friend: after 3-4 days (he might be away on a short trip)
14) a next door neighbour: after 2 weeks (possibly he's just on a vacation)