As it says on the tin, what are the differences between Young Adult and Adult life stages? What can one do that the other can't?

Forgive me if I'm blind and it's stated somewhere...

I don't know of any at this time. YAs are just younger adults.

Well, they seem to have different (albeit subtle) skin, particularly on the face area. Their facial shape itself also seems to change, some cases it's barely noticeable, others it's pretty drastic. Adults seem to have a slightly different body shape, but perhaps that's just women. Their breasts slightly sag down, and their hips are a tad wider.

I also want to say that they have a slower metabolism (gain weight faster/easier), but don't quote me on that.

I have a feeling that there's also a change in fertility for YA and A, but that's only from playing and I haven't taken a look at the tuning for that so don't quote me.

Actually, now that you point it out I think you're right about this. Once they go from YA to Adult, they do have to work a little harder to stay in shape.

I've noticed this as well, but there's some other factors that might be at play, such as baby weight(generally, my female sims finish up with their babies right as they're aging into adult) and cake weight(between the YA->A birthdays and the birthdays of the kids, that's a lot of cake to eat!). I'm not sure if it's hardcoded into the game, or if it's a difference in how I play the sims when they're YA or A.

Just like real life.

I've noticed the very subtle body changes only when I switch between YA to Adult. I wanted to find a way to tell Teen, YA and Adult apart easily in game but there doesn't seem to be when you haven't met them. I only find out a Sim is adult when my newly YA has fell in love with them and moved them in. Grrr.

I agree with the fertility (it was harder getting my Sims pregnant at all when they were adults). I haven't noticed the weight gain because my sims were like YoYo dieters lol

That's probably more for sims that are already in game when you install? In CAS you can give a teen/young adult/adult female sim almost non-existent hips and the perkiest breasts you've ever seen. It's all in the sliders. That most likely applies to elder female sims too.

I haven't run into any teen sims in game yet. In CAS you can tell the difference in the face- teen faces are smoother looking. The sliders can help you make the features more defined and more adult looking, but that smoothness doesn't change. Young adult skin doesn't look quite as smooth. (Adult skin has a few wrinkles and elder even more wrinkles, but they're neither teen or young adult so they don't count here, lol.)

Along with the "Ask if single" interaction, we need a "Ask age" one. Perhaps a bit easier to get a negative reaction if the sim is a full adult. There'd still be some risk...someone could be at the last day of YA, but you're less likely to hitch your 1-day YA to a 1-day-to-Elder.

exactly, what is the difference/ it already hit me with ts3. what is the use of young adults/ in ts2 they went to college. and it made sense to me.
but young adults without universities/ preposterous