Is there a nicer way to phrase this?

Sometimes you just have to say it's none of your business! My mother thinks she should know every detail about what is going on in my life. I will sometimes tell her "I am not discussing that with you".

i deal with someone who asks very pointed questions that i feel are "fishing" so i give a vague answer and re-direct by asking her a question back. i admit, before i speak to her, i mentally run over a few questions in my head so i am prepared to change the subject.

I agree with the others that you can redirect and keep on redirecting.

But something you said puzzles me. These are close family relatives and you think when they suggest teens/young adults should go back to school they are being ridiculous? I'm thinking here, you can't have it both ways. You can't have close family care about your kids and expect them to not want to know what kind of education and careers they have. To me, a ridiculous bit of advice would sound like "you should tell Johnny that he needs to trade in his new car for one that is a different color." Even with that, Grandma is perfectly reasonable in having the opinion that red cars should be avoided. She just can't expect her opinion to be taken as gospel and used for advice.

I like the suggestion to give vague response of "They are doing OK"

There is always the response of "Why do you ask?"

My standard response is, "I had no idea we were close enough for you to ask me a question like that." And then I just go about my business.

well, and I'm trying to say this gently myself, if the "child" is old enough to make such a life-altering decision like that (not finishing a degree), then he is old enough to talk to Grandma/Auntie himself. You don't need to say anything. Let him handle it. 

First, I think how annoyed things like this make you is a function of your (meaning the general public, not you in particular) personality.  Some people are annoyed by this, others, like me don't mind at all.

I tell my mom everything and don't mind her asking about things.  My sister is the complete opposite and claims my mom interrogates her. It makes my sister angry.  My mom is exactly the same with both of us so the difference in how we feel is based on our own personalities.

Now, like most mothers, she offers her opinion freely.  If she offers one I don't like or that is unsolicited, I usually joke with her and say something like "funny, I don't remember asking your opinion".  Or I just roll my eyes at her and she gets the message.

In your situation, I don't think getting snarky with these people or saying "mind your business" is appropriate.  A better way might be to is either simply tell them that everyone is "fine" and "not doing much".  It's short and gives no info you don't want to give.  With opinions, you can do like Siri does when people say she's stupid:  "Thank you for your opinion". 

ETA:  Free Charlie, I think you and I were hatched from the same egg.  We see the world the same on just about every situation.

"She's happy with her choice and I'm not paying her bills so I don't make the assumption my opinion on the subject matters."

Then move on. Truly, move on even after you've left the conversation. You've said your part now be done with it.

I'm sure it's not the case where you are concerned but I'll throw it out there for conversation sake. I have a relative who is always distraught and frequently says "omgosh what should I do?!" But then is furious anytime anybody says something she doesn't agree with or follows up with a general how did that turn out kind of question. If you truly don't want input, make sure *you* aren't putting someone in the position of giving it.

Honestly, no, there isn't a way to get topics dropped without offense IF the person/people asking are lifelong nosy people who haven't taken the hints you have already dropped. 
I assume you've already tried the "I don't know they lead their own life and don't tell me much" or "son is happy with his job" etc

From there it just gets stronger, more confrontational to "advice was asked"

At one point I had to tell me mom flat out that her unsolicited advice made me not liking to talk to her. Then when she gave unsolicited advice I'd say "thank you for an opinion about my life I didn't ask for and would hang up or leave"  Yes, it hurt her feelings. But my feelings also count and I still don't like unsolicited advice.  Now she asks if she can give advice. Sometimes I say no I am sure of the decision and sometimes I say ok.

All that to say, sometimes you just are going to have to offend to get the point across.  It will help with the kids move out and you can claim having no knowledge. But also let the adultkid drop the relationship with the extended family that can't hold their opinion to themselves.

'Why do you want to know' usually does the trick I find.

I usually just say, "ya think so?" And if they add more I say, "interesting..." and that's it. I couldn't care less what most peoples' opinions are.

If this is family then personally, being too blunt wouldn't really be ideal imo. I don't think taking an interest in the youngsters is wrong, that's what families are all about isn't it, but when they offer their unsolicited advice I'd try and find a non-committal way of responding. Maybe just smile or think of a vague "I'm sure she's happy doing what she's chosen to do" type of answer.

I think it's very telling that you consider questions from close family members about your adult children - someone intruding on your business.

There's generally more of a whammy effect if the question is very personal, asked by someone who has no business asking it and it's obviously overreaching. It could still work in this instance as well by responding to their answer "Well, I'm interested in your kids" with, "If you were truly interested in them, you would be asking them directly yourself." How many of them are going to go out of their way to call or see the kids in person so they can ask their questions directly?