> It could also be argued that 16-year olds could have been given a say in their entire future
Having chatted to a few of that age group about the subject, I agree. That generation is also more flexible, more mobile than the last, so I doubt they would think twice about leaving the UK in pursuit of better opportunities abroad.
I mean, if I was 16 again, moving to the EU for Schengen access alone would be highly attractive !
> the euro and schengen
The UK was never going to join the euro, that was one of the major carve-outs. There are also other EU countries operating outside the euro and highly-unlikely to change in the foreseeable future.
Technically the UK was not in Schengen, sure, again it was another UK carve-out. But with the EU passport there was no practical difference for the holder, you had the same freedom of movement in the Schengen area as a UK(EU) passport holder.
Leaving the EU also meant loss of access to EU security databases etc., which is why "stopping the boats" is harder from outside than within.