Do not worry about being an Engineering Officer yet. Really. Concentrate on getting yourself through IOT. Don't think past there. Put everything into getting through. I was 23 when I started IOT so was older than a few, but younger than a few - and the ratio of older has increased. Much of it isn't difficult to learn - but the quantity is high - especially to a non-military chap. Work on endurance fitness - you're going for some fairly long runs and you'll be tired before you start them from bulling the night before. It is an artificial environment but it has to be - it is training. Jump through their hoops and get to the other side - then worry about your Branch.
Good luck - best time of the year to start - weather is awful but gets better throughout your course.