Okay, just had to ask....
You are definitely old enough to understand rabbits and take care of one. However (I'm gonna sound like a parent here) unfortunately part of life is "doing what your parents tell you" right? Lol.....
But, with that being said, I can see both sides.
For one, your mom is absolutely right...rabbits do poop and pee aLOT and they are messy. Even one. I can't go one day without cleaning up after Agnes otherwise I smell her litterbox and her cage gets a bit messy. Rabbits also dig, chew, rearrange, throw hay everywhere, they are, well, messy. When people ask me if rabbits smell I always say that they themselves don't smell but their pee does and if not kept up on it can be a nuisance.
Now, that's not to say rabbits don't make wonderful pets. I love Agnes dearly but she is alot of work. Rabbits definitely don't make good let's being stuck in a cage 24/7 and forgotten about.
And I'm not saying you will or won't do this. You could be very serious and passionate about taking care of a rabbit. But I don't know you and I'm not your parents
have you had pets before that you "promised" to take care of but didn't? Do you have school activities or social time with friends that keep you busy? And what about college in a few years or moving out on your own? Rabbits can live for 10 years, sometimes more. Would you take bunny with? What about food, litter, hay, veggies, vet costs.
Now again, I don't know you and you could have the best intentions here but you do still have to answer to your parents. That's life!
if you really want a rabbit and are serious about taking on the work, than do your research on all these issues and bring the facts back to your parents. Show them this is important to you and that you know what is involved. If it is the smell your mom is worried about, tell her how you will tackle that. Look into the cage size you will need and the cost. Supplies, food, hay, veggies..tell her how you will cover that. Where will you keep the cage and the bunny? Is that area bunny proofed? Etc, etc, etc.......
I'm rooting for you if this is something you really can commit to. This is a great forum with lots of info to learn about rabbits.
Best of luck to you!!