Who goes to Uni?

Most likely young adults.

I'm hoping its YA+
I want to send adults and elders to uni too!

Hopefully all my YA's.

I generally send every ya I have in my Sims games to Uni eventually. I really like playing University.

I don't expect it but I would love it if teens have the option to go. Then I don't have to wait for a highschool mod.
One thing that kind of annoys me about this game is how they don't let us be in control of everything. In most countries young adults go to university but not in others, so if it's our game I think we should atleast have the option to send teens if we wanted to. It would be nice.

Just like those special lots and destination worlds we can't live in. I think it should be up to me if I want to live on/in it or not. I had to use a mod to live in chalet gardens. ? and if they give us a rental lot trait we can make any world a vacation or livable world to our choosing.

YA, I should think with the possibility of above that. I doubt it would be teens because that's what the senior school is for. Lets be honest, yes in real life, most people will be about 18 or 19 when going to University but that's largely classed as young adult by that point, really. The teens in these games are probably designed more like 14, 15 or 16, something like that.

I actually bumped into a group of Year 11s in my local supermarket recently. They would only have been about 15 or 16 but they could have passed as adults. Looking at them, I don't think having the teens in Sims 4 looking like the young adults was actually a bad thing. A bit more realistic, if anything.

Teens already have high school, why would they get uni when they never got in the past

Hopefully YA A end E can go like a real uni.

As far as I know colleges require you to be YA (18 and older to attend) so the age is YA at least.
I also understand if one wanted to rent a house or apartment not belonging to the University - people need to be 21 in the USA and hotels, B&B's, etc can not rent even a room to a student under 21 who is not accompanied by a parent or guardian - which I find odd seeing 18 is considered YA in USA anyway, just not if one wants to rent in a rooming house, hotel, or B & B.

As the game tends to follow the laws of USA - I was surprised to learn this about hotels, rooming houses and B & B's and other rentals off campus. But it explains why college students have to stay in dorms at USA Universities until they are 21to live near their University.

ETA - From what I can see it depends on the state as I have been looking around at other states and not all seem to have this - unless it is just near Universities.

Should be YA+
I wouldn't want an in-between just for this, seems unnecessary.