There is always a list of requirements that a Sim must meet in order to be picked by the game. That’s why children would visit libraries but not nightclubs. Besides age, the requirements could also include traits, skill levels, fame level, occult type, career type, whether they’re currently at work/school, whether they’re played or unplayed, what world/neighborhood they live in, whether they have spouse/children or not, etc. Each venue type has their own list of requirements. Some venues could have multiple sets of requirements for different types of visitors (for instance, art galleries attract both regular patrons and painters; these two have different requirements).
Besides the hard requirements that must be met, there are also criteria that make certain Sims more likely to chosen than other Sims. For instance, a Sim with level 5 fitness skill is more likely to be picked as a gym goer than a Sim who’s only level 1.
When picking a Sim, the game goes through every single Sim in your game and check whether they meet the hard requirements or not. If they don’t, they’re instantly tossed out. If they do, the game will calculate a score between 0 to 1 for this Sim based on how close they are to the the favorable criteria, with 1 being a perfect match. Once this is done, the game will then use weighted random to pick from the five highest scored Sims. Weighted random means that if Eliza Pancakes receives a score of 0.8, and Summer Holiday receives a score of 0.4, then Eliza is twice as likely to be picked as Summer.
