I've had a few times where it seems as if there are no new sims, the "old" families children have become adults but none have married or had children. So, fresh blood is needed. Yet none seemed to be added to the town, or they are adults or elders and are of no help for my teens soon to age up to YA.
I took care of that by starting a new game (really, several games), creating a sim and moving him or her into a starter home, give them some money by the kaching cheat, copy to bin and then when I want fresh blood, new bodies, I just take one or two from the bin. Also, save families that I play in other games where they have children and add them to the town so my sim's family with children will have future mates when they all reach YA age.
I seem to have the bad luck of playing a sim family in a town that doesn't have anyone my sim's offspring can be mated too. Either there is no one their age, or they are so different it won't work out. I am too picky, probably in what I want also. I don't want, at least for right now, to have my sims be mated to a sim with any of the traits of insane, neurotic, klepto, evil or mean.