Being an "adult" is an ambiguous thingt. It isn't a state, as it can change. Adulthood is defined by laws, experience, choice, perception to name a few. 

Legally (US) an adult is not well defined.

• At 16 you can get a license and drive a car, a huge responsibility.
• At 18 you can: vote, sign contracts, join the military, by a long gun, gamble...
• At 21 you can drink alcohol and buy and handgun
• At 25 you can rent a car and become a Congressmen.
• At 30 you can become a Senator
• At 35 you can run for President

Being an adult is like porn, "can't define it but I know it when I see it", -some Supreme Court Justice

This system is asnine! At 16 you can drive a car filled with other 16 year olds on freeways filled with other people at 70mph, but you can't buy a lotto ticket or a cigar. At 18 you can join the military, be issued weapons and permission to kill, but you can't have a beer or buy your own pistol. You can get sent to war but not vote on it. Children are children, but they can be tried as adults if they commit a crime.

Two Options

The easy option is to choose a single age. The better option is to take an "adult" test.

Normalize Adutlhood

At a certain age you are an adult, be it 18 or 21. At that age all rights and priveleges are available to you. Misbehave and you can lose them. With power comes responsibilty. All the obligations of adulthood come with the power. Debt, punishment, etc.

Test for It

At any point from 16-21 you may opt to test for your adulthood. The test will not be easy. It will cover subjects like, but not limited to:

• debt obligations (including interest rates, late payments, etc)
• firearms (safety, legal restrictions, etc)
• contract law (did you read the fine print?)
• civics
• the penal system

The test should be free, becoming an adult is not for the wealthy. Because the test is long and arduous it will prevent people from taking it when not ready. The test should be 200+ questions and take 1/2 day. It is a once-in-a-lifetime test.

Once you become an adult it is final and covers everything. Colleges can't ask for your parent's income (unless they cosign). Gambling, voting, and drinking are activies for adults only.

Example

Under New Jersey law, a non-custodial parent's child-support obligation generally ends when the child turns 19. But if the kid goes to college, a court can order the non-custodial parent to continue support through age 23. Aggrieved dad sues, arguing that it is unconstitutional to require him to pay extended support when a still-married parent could throw the kid out on her ear the moment she turned 19. Third Circuit (unpublished): The state says children with divorced parents have a harder time affording college, and the state can rationally try to remedy that.

https://reason.com/volokh/2023/10/27/short-circuit-a-roundup-of-recent-federal-court-decisions-234/#more-8253130

Under New Jersey’s child support law, a noncustodial parent may be ordered to
provide financial support for a child past the age of majority to attend college. Larnado Pittman was subject to such an order and challenged the law on equal protection and due process grounds. We will affirm the dismissal of his complaint

https://www2.ca3.uscourts.gov/opinarch/221435np.pdf

age of majority: The age of majority is the threshold of legal adulthood as recognized or declared in law.

In other words (aka plain English), an adult.

A (non-custodial) parent is financially responsible for an adult until the age of 23. Weirdly, only as far as advanced education is concerned. The 23 year old "child" can starve, be homeless, lack medical care, etc., but the non-custodial parent must continue to help pay for 10k-50k per year advanced education. This would include, Liberal Arts, Music, Drama- but would it include trade schools?