I think it completely depends upon the person. I'm by far the most sensible of all my mates, and always have been.
When I passed my test, I was chomping at the bit to get out in a car. I didn’t buy one immediately – I had access to my dad’s car. He had a works van and goes down “the club” (British Legion) most evenings for a couple of hours. As long as I was willing to give him a lift there and back, I had unlimited access to his car. I was given the impression that I would have to go out under my dad’s supervision a few times before he would let me loose with it on my own. For 2 weeks I harassed him about going out with him in the car and then finally he just handed me the keys and told me I could go out on my own. He probably just wanted me to get out of my system that “just passed my test, I am a great driver/I am invincible” buzz everyone gets when they first pass. He knew I was sensible enough to handle it. It was by no means a pootler. Unlike all my mates thrashing their mam’s Rover Metro, I was let loose on a 2.8L Ford Granada estate.
It is character building to start from nothing and working your way up through bangers you fix yourself etc to the good stuff, but a lot of it will depend on the person. Some people are dicks behind the wheel when they are young and grow out of it, some people will be dicks forever, some people will have their sensible heads on from day 1.
Everyone wants a nice car at 17, but not everyone is desperate to leave home (I was – it was a powerful motivator not to spend too much money on a car). If you’re happy living with your parents, financing a GTI/GTI probably isn’t going to break the bank (although the insurance might)
There is no waiting these days – buy today and pay for it tomorrow, how many people here are buying their car outright? Car finance has never been so accessible.
I think my sensibilities are shown in my car of choice – a relatively expensive car on paper which is not as expensive to run as most would imagine due to the excellent residuals. I get to run a £26k VW for the same price as running a £17k Ford/Vauxhall/Alfa/Renault etc. That is smart car buying as far as I’m concerned.