I think you should seriously consider taking up the tuba and at age eleven now would be the right time to do it!! I know from my experience of playing with youth bands when I was a kid (not so long ago!) that there has always been a shortage of good young tuba players so you might find lots of opportunities to play in bands and orchestras almost straight away.
I reckon you should go for it and see how you get on. I think the biggest problem for young tuba players is having to carry the beast around with you on school buses etc. ........ bit of a drag, but if (like me) you joined a youth band who did lots of travelling all over the world, it really is worth it!!!
P.S. If you're quite a small eleven year old you might need to sit on a some cushions for a few years until your lips can reach the mouthpiece.