Okay, buckle in, this will be long, but I hope it helps.
My son developed a quite sudden ambition at 18 to do something he'd shown no great talent at, and frankly no real interest in either, up until he needed to choose a degree course. He had never expressed any real ambition or direction towards any particular career choice, so picking a degree to do was a bit of a chore, with no specific goal in sight. I steered him towards a degree in his favourite A level subject because it was a solid academic subject that offered flexibility for future careers and it was the only thing that made sense at the time. My logic was that it was a generic subject and at least he'd know what he was letting himself in for, having already studied it at A level and quite enjoyed it.
He then decided completely out of the blue that he wanted to do this particular vocational 2 year college course instead. Let's say for example it was 'X Box game design.' It wasn't, but that's a good example of the type of thing. I apparently at some point said 'but that's not a proper education.' I don't remember saying this, but he still hold it against me more than 10 years later.
The issue for me was that in order to get onto this 'XBox game design' course he would have needed to demonstrate a competence in IT which he was frankly didn't have. He hated IT at school. He liked the idea of 'designing XBox games' purely because he liked playing them, but was ignoring the fact that it was a very heavily IT based course, which was not his forte.
He'd also have to show a portfolio of work in the related subject, which he didn't have. Although the course didn't require A levels to get onto it, he didn't really have the specific skills they were looking for, so he would have been over-qualifed but in the wrong things. I felt that, even if by some miracle he was accepted, he might not enjoy or cope with so much IT based content.
He then said to me that he'd never wanted to do A levels anyway and he'd wanted to pursue 'XBox game design' since he was 16, but I'd not listened to him. I honestly have zero recollection of it ever coming up in conversation once. I think he was just panicking about uni, feeling pressured and was clutching at straws.
He ended up doing his degree in the suject I'd suggested, but hated it, and he did for a long while really hold that against me. I feel terrible that he didn't feel supported in his sudden 'ambition' and that I didn't take him seriously, but honestly, I just thought he was plucking this 'XBox game design' out of thin air for want of any better ideas at the time. I'd have supported him in doing absolutely anything, if I'd known it was a genuine passion or ambition and something he demonstrated some reasonable ability in. He thinks I was just obsessed with him going to uni and wouldn't take no for an answer. I admit I can see why it might have seemed that way to him at the time.
To be fair, he never actually said he didn't want to go, but I should have recognised the signs that he was having cold feet.
With hindsight, what he really needed, and what I should have suggested, was a gap year. He could have spent the year gaining the particular skills needed to pursue the 'XBox game design' thing if he was serious about that, or just take his time to think about what degree was right for him, or look into other options like apprenticeships. I regret that I didn't recognise that at the time. I also regret that he never felt he could come to us and tell us he was unhappy with his degree choice. We would have understood and supported him switching courses, even if it put him a year behind and cost us money. But he never said anyting at all. He was always so private, didn't open up to us much about anything and was not one to phone us often anyway, in spite of a good relationship, so we had no idea he was unhappy in his course. I'm not sure if he even realised that switching courses was an option, or whether he felt he'd have been fed up regardless because he didn't want to be there. He did graduate, but was in a bad mood for two years afterwards.
However, several years and several low-paid, non-descript jobs down the line, he is now doing really well in a management position, in a great company with real prospects for progression. His degree is irrelevant to his job and he ended up in that job entirely by accident, having gone in as a temp, quickly being spotted, recruited permanently, then promoted, twice in three years. He liked it there, they liked him and it just fell into place. It's certainly not the sort of job he'd have had ambitions to do at 18, but it's a solid career and he's loving it.
I don't know sometimes whether we should keep our mouths shut and watch them make mistakes, but at least it's their mistake to make, or to try to steer them towards what we think is best, only to have them resent us for meddling. But I think the advice I'd give is to listen more closely, and to read the unspoken signs. I think my son was giving off massive anxiety at going to uni and I just didn't want to see it.
Let them know that at 18 they have the time and space to try something and if it's the wrong choice, it's not the end of the world, there is time to change tack and try something else instead. You will understand and support them either way and they can always talk to you if they are feeling unsure or unhappy about their decision.