How can an under age person get an apartment without parent consent?

How is this underage planning on paying the rent, security deposit and bills?
What is the age and state?

They can't. A minor can't sign a contract and lease is a contract.

What if a older brother or sister is willing to sign for them?

It isn't a question of 'can' or not; it's that a minor cannot enter into a legally binding contract, so no landlord with any sense is going to rent to one. You could fail to pay rent, damage the building, cause all kinds of problems, and the landlord would have to go after the parents.
Some states allow emancipation of minors if they apply to the court and prove their ability to support themselves, but it's rarely granted.
If you wrote an app that is raking in a dollar a minute, knock yourself out.

Some states you can get emancipated but you have to have your parents permission and you have to have a job and some other qualifications. Your older sibling could only sign if they were moving in with you. Then you very likely would have to be mentioned on the lease as living there as well. Again you would need parental consent.

A minor is under the control of their parents or legal guardian. Only the legal guardian can sign for them. And a landlord is still going to be reluctant to rent to a minor. Even if the guardian claims its for them the landlord can evict them if they find out.

... Your older sibling could only sign if they were moving in with you. ...

Well, that would be up to the landlord. But otherwise, I agree with what everyone else has said.

There is no legal way for a minor, who is not emancipated to rent an apartment.

You may find some real "dumps" or places rented by what I call "sum lords" who ask no questions, want to be paid in cash. We all know there are some people who rent without asking questions,

But the real issue, is as a minor, you can not even move legally from parents home. Anyone helping you could be commuting a criminal act