Yes. Under 18, by law. Under 30 or so if one is thinking of emotional development.
PHYSIOLOGICALLY IMMATURE
Are you with a straight face asserting that a 17 year old human is NOT physically mature?
Their outward appearance might not change much after 17 but their brain is still developing, which in turn is reflected in their behavior. Current medical research indicates those changes are pretty much complete at 25 years of age but might take a bit longer in some individuals.
Yes, I'm aware. Thus the term "physiologically".
Brain isn’t physiologically?
Sure sure, having a brain injury is totally going to make your bones shrink in size and change your hair color, right?
Honestly, do you not understand the distinction between somatic and psychological? Your house has a foundation and a physical structure. It also has paint and furniture. All of those are "physical" things sure, but the structural components of the house is what makes it a house, the decor and design gives it personality.
Toni and Arctish are also making a bit of a heap fallacy here. Cognitive development is tied to physical development, and it's true that a 17 year old isn't completely cognitively cooked. But a normal 17 year old is far closer to a 22 year old in terms of cognitive capability, complex thought, executive function, and extrapolative ability than a 12 year old. Cognitive development is a continuum, but it's not a uniform continuum... which is the pretense embedded in Toni's argument. There are developmental stages in humans that are slow processes of building information and compiling knowledge, but there are also stages that include massive leaps in functionality. This isn't mysterious or unknown, and I'm irritated with people pretending that it is somehow magically unknowable.
Very young infants have fairly low cognitive development. For the first few months, almost all of the energy goes to physical growth, and the brain mostly stays focused on "eat, sleep, poop". There's still some cognitive development, largely focused around the parental bond and facial recognition processes, the brain learning how to control eyeballs and process visual and aural stimulation. Then the process shifts priorities for a while, with a change in the inflection of physical growth - more energy investment goes into the neural functions for controlling the body (sit up, crawl, walk, grab, hold, climb) and language acquisition (non-verbal indication and expression of emotion, toddler jargon, basic vocabulary and syntax needed for core communication). Around 2, physical growth slows and enters a fairly linear increase. Between then and about 10, energy is split between increasing size overall and acquiring functional knowledge which is largely "stored facts" about the world around them and the people in it. Between 10 and about 14, height increases a bit more quickly, and the cognitive development of friendship bonds occurs. That's a key period of psychological development, where children start to think about other people from an external perspective rather than their own, and they start to internalize that other people like things that they don't like, and vice versa - they start to understand uniqueness of personality and interest, and that leads to compatibility bonding on shared interests and likes. When puberty hits, there's a huge increase in energy consumption, because there are several really critical developmental steps as they transition from juvenile to adult. There are lots of physical changes, including significant height increases as the long bones lengthen, accretion of bone density and strength, closure of the growth plates, and maturation of the reproductive system. But there are also significant cognitive changes - extrapolative ability and executive function increase substantially, and romantic bonding develops where the communal drives shift from compatibility of personality to compatibility of reproduction and all of the attendant sexual urges. The entire process of puberty takes a few years, but a huge amount of it happens in the course of the first several months. After about 16, the remaining physical changes are really minor in the grand scheme of things, and the cognitive development tends to be focused around executive function and emotional equilibrium.
To oversimplify an analogy... a feline that has just had her first heat isn't completely matured either physically or cognitively, but she's far, far, far more similar in both to a cat than to a kitten - even if she was a kitten just two weeks ago. The impact of puberty is enormous, and it really shouldn't be understated.