Okay, so, one way to do this is by hiring a nanny.
Once the nanny comes, have the parents only move out.'

The game considers the nanny an adult on the lot so it lets the parents leave and then dismiss the nanny and THERE you have a teen only household.

Here's the way I do it, without dealing with parents at all:
Make your teen Sim in CAS the way you want them. Just before saving the family, turn them into an adult.
Move your Sim onto the lot.
Use the tool of your choice (InSim/Sim Blender/Sim Manipulator, or I believe one of the built-in debug tools can do this) to age them back to teen.

I suggest grabbing Christianlov's modified clothing rack and setting the options to free clothes if they age down into something a bit weird-looking.

You don't need the nanny.Teens can be alone on lots without parents, so moving the parents out and undoing the relationship with a tool is enough.

When I made the Warehouse Boys, I made them as three groups of adults, moved them onto the same lot, and then aged everybody down with the cheat, or it might've been the debug tool, I don't remember exactly. Just be aware that if you do this, they'll all have F grades. Which, again, is fine for teens; but if you want a mixed teens/child household ala the Newsons, you'll need to use a tool to change the children's grades if you don't want them taken by the Social Worker.

I didn't realize that about the F grade. No wonder my self-Sim kept coming home with horrible grades despite completing her homework every day.

I had a child in the household the time so maybe that's it.

Teens can look after children, too.

Bulbizarre, that sounds like it needs troubleshooting. The Warehouse Boys started off with Fs, but they were able to advance their grades normally.

No, you don't need a nanny. In Baldrair Bluffs, Alastair Gunn's parents moved out while he was at school, so he came home to discover that he was now living alone. (It was some madcap idea of his father's to make him more self-sufficient.) To add insult to injury, his parents phoned him up at 4 p.m. to complain that he wasn't keeping in touch!

My game is heavily modded, but I'm pretty sure that this would work even without mods.

Alastair actually did alright. He invited some friends round to ask their advice, and one of them, an intelligent Knowledge teen called Claudia, eventually moved in and became his girlfriend. Though they are still teens, they have now adopted a little girl through the Social Work Department.

It seemed like it worked pretty well, then.

I was thinking I didn't have that problem when I aged the Singles girls down. I did have it with another teen, who had been born and grown-up in the normal manner; I don't know what the cause was but using the Homework Table fixed his grades.

I phrased that a bit wrong. My self-Sim's grades did go back to normal after a few days, but I didn't realize why they were so low to begin with. Seeing "Bulby is improving in school, but still has horrible grades" was a bit confusing. (No, I didn't actually name myself Bulby. )

One of my neighborhoods has a few teen-only households.
In one story... the Apocalypse wiped some adults out and made them into tombstones or townies/downtownies.
Via the Sims Shrub Hack selector from (I think) Boiling Oil.
The teens control and regulate some of the community lots used for trading goods -- since money is no longer of value.
Even though households have 'cash' in their finance tab --- the N-hood is played like they have to "barter" for goods.
The Apoc. sims have a tough life. :p

Did you have a link for that ? Or if it's here... which thread ?
Thanks in advance for any help.

Child/younger are the only ages that require supervision in shipped version of game. though if you did want a Child (or younger) to be the oldest household member, I imagine a Nanny would be a temporary solution; also Butler (if Apartment Life is installed).

Teens. the mentioned "debug tool"; I think it is the one shaped like a baby; I do not remember the name. an alternative to that object could be the agesims cheat (agesimscheat on).
another option to make Teens oldest could be to convert Adult/Elder to townie types through various hacks.

The homework table is by Inge on Simlogical here.

I play a lot of teens living independently, and find that Adults aged down to teens using the agesimscheat cheat start off with an F grade, but, if I use the Sim Manipulator to age them down they usually start with a C grade.

When making households of teens in CAS, I sometimes start them all as adults and age them down the way Peni described above. At other times I make all but one of them teens and make the other one an adult and the parent of all the others. Once I have them in game I remove all the family ties (and Family tags) and age the ex-parent down to a teen too. This means that most of them can be given their final appearance in CAS, but I find I have to use SimPE as well as the Sim Manipulator to completely remove the family relationships.

Of course in Alastair Gunn's case, I didn't remove the family ties, as his parents are alive and well, and have merely moved to a nice new bungalow further up the hills.

[EDIT] I see I've got a Disagree almost as soon as I posted. Have I made a mistake? There were originally a couple of typos in my second sentence, but I've corrected them.

For quite some time now, I've been using the Apartment Life compatible Teen Move Out mod found at http://gardenofshadows.org.uk/garde...18434#msg718434 to have teens move out on their own, and even rent apartments on their own...without having to age anyone up and back down again, or any other tricks or added mods.

With this I've had a few teen sims move out from their parent(s) homes (they end up in the neighborhood family bin like normal) and then they were moved in either a small home or able to rent their own apartment. The school bus and carpool will still show up like normal unless you prevent it using some other modded method. So they can still be teens, but on their own and still keep their family ties/relationships.

I rather like the challenge it brings having a teen leave home early and have to try and maintain their schooling, earn money and pay bills.

I create adult sims in CAS and also use BoilingOil's no nanny mod, just in case there are younger sims in the house plus a mod (forget by who but it's here on MTS I think) for Gypsy to bring a genie lamp, modded for adult wish to age back to teen. This way there are no extra sims to kill or make into townies and send off into the unknown. Now the teens are in charge, can go to school, get a job, take care of siblings or whatever teens living alone can do. When you find the mod, read the entire post, aging down can cause loss of a few things. They won't lose any skills gained but there are some things they do. (:

I think this would be a really cool idea for the sims I don't end up sending to university! I could move them out and turn their aging off until their peers get backs they'd be age synced but it would almost simulate being a young adult.

Age down the mother and father to teenagers, then kill of their teenage son/daugther. Then make the undesirable choice of kicking one of them. Congrats, you have a household with just a teen!

I just make them Independent Teens with InSim. I haven't had any trouble with that but it does allow them to have adult relationships, which is usually what I want. So, really, they are adults in teen bodies that way.