I don't really play young families so I don't really have an opinion on that. But I think that Young Adult / Adult / Midlife Adult / Elder might need a better transition.
Young Adult can range to full time work, full time post secondary school, or part time work/school, etc. Either venturing for a romantic partner (and settling down) or start playing the field. I'd say Sim wise this is a idea time to work on vocations/skills/starting career(s) while trying to also balance friends/lifestyles.
I think that Adult stage shouldn't get the midlife crisis as it feels in game Adult is what 30s-40s? Which in this day and age isn't exactly considered an over the hill anymore, sure they might start getting grey hair but in some cases the baby fat (young/youthful full face) starts to look defined in a good way. For some that is the age when they are finally financially able to afford a house (well if lucky), and have a family. But, yes, in some cases can start to become grand-parents. But generally Sim wise by this stage they are skilled and career is fairly advanced by now.
Midlife 50s-60s would be more accurate for a midlife crisis, and finishing going grey and starting to go white. And can be grandparents, can retire early, and health problems can start. And maybe some might get cosmetic surgery or treatments.
Elders/Seniors 65+/70+ and should be retiring/retired, may need walking sticks/walkers, may become great-grandparents, etc. However, I like this to also be a linchpin/matriarch/patriarch established family/dynasty member. I'd like them to not just step down from their position because there are younger members. I love the idea of matriarch/patriarch family/career leads that humble others either with wisdom or fear!
Adults in TS4 are about 40-60.
I am skeptical of that because most people wouldn't regard 30s as young adult, at most 18-35 but not all of 30s (and nasolabial lines tend to start in late 20s/early 30s and that is the most physically defining feature of an adult in the Sims game). And people can take early retirement as young as 50s (but also working in their 70s). Hence, my reasoning for adding a midlife adult.
