Why Can't I Adopt an Elder

Probably because adoption is mainly focused on children and under. Yes, teens are adopted in real life but not in this game I guess.

In the Sims 4, You don't adopt elders, elders adopt you!

I delete them. But my game seems to generate a ridiculous number of elders.

It's just sad to think of all of those grannies and pappies sitting around wondering where a family to love them may be. And the poor misunderstood emo teen who didn't mean to catch the house on fire while playing with matches and gasoline in his bedroom. He was just bored and underappreciated for his natural talents. So many sad stories, so few ways to adopt them.

You can always add them in manually through World Mode. But if you want to roleplay, you can always make a retirement home/elder pound and place all of them there. Then, whenever your sims build up a relationship with them, they can invite them into their home.

You can ask grown-up sims to join your household. All you need is make a high relationship level to them and then the option 'Ask to Move In' will be unlocked.

Yes, why can't sims adopt teens, adults or even yes, elders ? An elder could also adopt an adult.

Lol y'all are missing the point. First it was a joke. Lol Second, I said like infants, toddlers and children. In otherwords, some random teen or elder pops up at your house and just moves in. (I havent' adopted in Sims 4 yet) but presumably it's handled by a case worker and you have no choice in what kind of person you wind up with. :p

When they do show up, can't you just ask them to move in, after the relationship is high enough?

Okay, so here's the scenario:

You're sitting in your livingroom on the big day. You know your new grandpa and teenage son are going to be here soon and can't wait to meet these dear sweet people.

Then, the door opens and in walks crotchety old man. He walks up to you and says "that's my chair. Move I want to sit down! What is this rubbish you're watching on T.V.? Turn that garbage off and turn on the weather! Back in Sims 1 we didn't even have weather. It was the same day and night. Come to think of it, I don't even think we had night! You kids today have it too easy! This place is a mess. If my dear Gertrude were still living, she could teach you a thing or two about housekeeping! And don't be feeding me any of that vegan nonsense! I want meat! Do you have any children? When I was growing up I was taught kids were to be seen, not heard! I'll not abide those little critters running amok! Now, keep it down, the commercial's over!"

Then a surly teenager enters in and you compose yourself and say "welcome to your new home and new family!"

He says, "whatever. I'll be in my room. Where are the matches?"

Granpa sees the teen and says "you are the spawn of Satan and this is now the house of the devil!"

The teenager says "whatever. So where are the matches?"

/Cut scene and welcome to your new life!