Mid-life crisis would be nice. It would also be nice if adult sims were more family driven. Focusing more on their kid's needs over theirs. Having whims that focus on bonding and building/growing their family, working on their careers. If story progression is added back adults could be the age that most sims would start getting married and having kids. As well as start getting whims to get married and have kids. They could also get sad when they see couples or kids as they get closer to becoming elders if they themselves don't have someone or kids. Obviously non-committal and hates children sims wouldn't be affected by some of those.
YA would be more self driven. Focusing on themselves and what they needs. Having whims to experience life, have fun, make friends, and get into relationship. YA could be more likely to decline marriage proposal and be less willing to accept "try for baby". Having high romance and friendship would be the only way to really increase your odds of acceptance and family oriented sims would be more than willing to accept.