I think I probably decided that it was for me after I completed a year in industry for UTC aerospace in Plymouth. The year in industry scheme places you in companies around the UK for a year ofter just after you’ve finished school. It really helped me try it out and decide that it was what I wanted to to. https://www.etrust.org.uk/the-year-in-industry
For me, it wasn’t a sudden decision. Whilst at school I enjoyed maths and science (especially physics), so did some research on careers related to those subjects. Engineering sounded pretty cool, so when I was in Year 10 I looked to complete my work experience at an aerospace company which I really enjoyed. So maybe at that point I thought “I’d like to be an engineer”, but it was something I had considered earlier.
I think similar to the other guys who have already replied, it is something that I realised gradually. Only after visiting some different sites in the UK, like for me… HMS Belfast and the JET fusion reactor in Oxford, did I think making stuff like this would be cool! Then I studied engineering because my next door neighbour, who was a civil engineer, told me it was the best way to make practical use out of my enjoyment of studying and learning about science.