Okay so...
It may feel like that at first, but you may learn to love it? You could look at the aspects of each subject area and lists its pros and cons (I know I did this for CS and Maths). My question is what made you want to purse arts? Was it the job prospects? Did it line up with what you wanted at that time? And have that changed? That feeling of unsureness, could it be from knowing that engineering has good oppourtunities? Ultimately, it's doing something you feel deeply passionate for. And I think to determine that is, would you go out your way to learn more about that subject (like as a hobby)? For example in maths, reading outside of a taught area.
It's great that you did really in your A-Levels, and is very capable of doing a STEM subject. That's great, but don't let that discourage you in trying to pursue arts. Both equally as good. The only way to know if it is a phase or not, is to just take both of the classes. If your current university has a engineering course, then go to some of its lectures. See how it goes. You may find it to be different to what you are expecting.
Your family would love if you wanted to change course? I would say, just do what you want to do. At the end of the day, it is your life and don't let others influence into something you don't want.
The age doesn't matter. Trust me. I know 3 people in my maths class that are much older than the rest. And maths is a wildly unpopular subject, yet there are 3 peolple much older than the rest. Don't feel discouraged about this, we all take our time and each have different paces.
And from this, I wish you the best of luck in finding what you wanna do! Remember to just take your time, no need to rush!