Here's what I have worked out so far. When it comes to specific sims visiting lots, it seems to be random, often they are friends of your played sim. Probably to save memory.
Street - In the morning, Teens, and Kids (Who are going to school, and YA and A who are going to work, and joggers), During lunch time - elders, because they have no jobs, in the afternoon, teens and kids, going home from school, and YA and A coming home from work, and then during late hours, YA and A in party wear, because they are going out for parties, and late during the night, teens, who are out and doing mischief I guess.
Gym - YA, and A who are often active. Other than that, it seems random.
Library - Random, but mostly kids.
Lounges - Lately, they have seem to be more random, but they used to have a lot of elders.
Bars - YA, and A
Parks - Random, but a lot of Teens hang out here at night, if you have trouble finding teen friends, go to the park late in the evening.
Nightclubs - Seems to be mostly YA and A.
Restaurants - The game seems to often pick families or sims dating or who are married to each other. But also random sims.
Cafe's - seem to be random, mostly YA and A.
Museum - No idea, seems to be mostly YA and A for me.
Pool - Mostly elders and kids
Spa - seems to be mostly YA, and A who also automatically gets tense.
