Well I asked for feedback so thanks to all that gave it 
Here is my unashamedly long winded reply; I feel it warrants a lot of words as, strangely, I partly agree and disagree with all of what's been said.
Sorry if it's too much but hey ho...
Yes it was a selfish and very inconsiderate thing he did and yes he behaved like a 'dick'. I don't doubt there are other far more caring younger children or far more mature young adults out there who wouldn't dream of acting this way. I agree his behaviour was thoughtless, self centred and totally unacceptable.
I also know I need to make sure he understands what the parameters of acceptable behaviour are; it's my duty as a parent and to society.
Then there the idea that the fact that I haven't done it before now would indicate I'm a failure as a parent.....erm I'll get to that later....read on if you wish...
So is he an adult or something 'less cooked' ? He is an adult intrinsically but with a whole deal of maturing and learning still to do. He's a work in progress....but then aren't we all to some degree?
I know from personal and professional experience that people reach fully fledged effective, responsible, independent adulthood (and all the emotional and behavioural maturity that it entails) at different rates. Its great if you (or your kids) can get there speedily, fantastic. Ultimately though what matters most is that you do actually get there.
I think its a bit like cars- different models have differing 'nought to sixty' ranges. Wouldn't it be wonderful if we were all Ferraris speeding along a smooth clear road, good weather, lights always on green with a competent and experience driver at the wheel. We'd arrive promptly at our desired destination without a hair out of place, looking sleek and stylish having accomplished our journey in a blink of an eye.
Sadly though in RL there are all sorts of vehicles on the road, some more cumbersome than others and some have trailers or caravans to pull. Hopefully they'll have the necessary engine capacity, if not they just have to work with what they got and trundle on. That 1.5L may not win the race but if it just keeps putting in the miles it'll make it to the finish line.....ok so there might be a few wrong turns and a bit of shouting at the blinking satnav along the way but he gets there in the end.
So to get to the point of it: this morning my son came to me of his own volition, showed he knew he'd acted badly and made amends in his own way. I could see he was genuinely sorry and was touched by his actions....all without me having to say a word. When we did have 'the talk' his reply of 'I'm really sorry Mum, I know I upset you and I feel bad,' spoke volumes.
Now back to failure: well there may be some truth in that, I know I've definitely made some mistakes along the way....haven't we all? Nevertheless I once heard an extremely successful and influential figure say ' The biggest guarantee of success is to accept the possibility of failure; once you accept that it's ok to fail if you learn from it and keep on going, that's the way to success.'
So I may have failed if, at 18, DS is still making this type of mistake but then from within himself, with no prompting, he saw the error of his ways, admitted it and tried to put it right....haven't we all done the same? He may not be a Ferrari, maybe its my fault he's not but in the end he made it .....even at 1.5L speed that's a win in my eyes.
Look for failure, that's what you'll find, be grateful for success and you'll bring more.
If you stayed with me to here then again thank you :-).......now go and award yourself some shiny new hub caps you defo earned them!