My sister and I moved to Scotland from a completely different education system (South African) as well, my sister ended up turning 19 while in S6 because the school year was completely different to our previous education system (which ran from January-December). I was also 17 in S5 and turned 18 in S6, then had to take several gap years before being able to go to university.
It was a massive change and a struggle to begin with (some subjects were easier, other more difficult), but my sister and I got a lot of support from our teachers, e.g. I attended every single after school chemistry help session run by the science department- I went from never having done chemistry before to getting A's in Highers and Advanced Highers with the support I got. It was massively helpful to be able to ask questions and have teachers explain concepts, even just discussing things with other students in the class could be really helpful. I know people do it online and on their own but personally I think it would have been a lot more stressful for me, difficult to stay motivated and I probably wouldn't have done as well as I did but everyone is different.
I needed to do Advanced Highers in order to study medicine so I had to stay but I am glad I needed to stay if that makes sense. I was still able to do volunteering in S6, I had medical work experience placements and was able to do an elective course on Anatomy through my school while in S6 which was really interesting. After high school, I went on to do more volunteering (because I was over 18, I was able to volunteer for an Autism Charity working with children), I also did a research internship with a psychiatry professor.
Even though I didn't apply to Universities while in high school, I learnt so much about the process, what I needed, etc through being at school and that helped me when it came time to apply. Teachers also got to know me more and several were happy to write references for me, my pupil support teacher also ended up being my referee for my UCAS application even though I had already graduated.
I would personally say ignore that everyone around you is a bit younger (I know it can be hard, especially if maturity levels are low), but when you start working or go to university it doesn't really matter, the people that I know on my course range in age from 17 to late 30's and I have heard of older.
At the end of the day, you need to do what's best for you. As cheesy as this may sound, a pros and cons list may not be a bad idea, get everything down on paper and then weigh it up.
Sorry for the long message, I hope it was at least a bit helpful.