At 11, you're certainly old enough to cook spaghetti--or pretty much anything else. I've taught children's cooking classes for years. My youngest students are 4-5 years old. I know some 10 year olds who have competed in the Pillsbury Bake-Off. When it comes to cooking, interest is more important than age, I believe. I don't allow my very young students to do dangerous things like cook spaghetti, or use sharp knives, but even at that age, they are capable of learning proper techniques for many cooking jobs.
However, that being said, one thing I always impress upon my students is that when you cook at home, you need to have the permission and supervision of an adult helper. If you want to make spaghetti, I'd recommend that you get the box of pasta out, read the directions--read them again, so you're sure you understand them. Then ask your parent if it's okay for you to make the spaghetti--and tell them the steps, so they know you understand what needs to be done. I would recommend that you ask them to drain the spaghetti for you. As someone else said, it can really splash around a lot, and you need to be of a certain height to pour it safely into a collander in the sink, so that's something best done by a full-sized adult, I'm afraid.
If you're interested in all kinds of cooking, there are some great kids or beginning cookbooks out there. Cooking Wizardry for Kids is an especially great cookbook for someone your age. It's got great recipes, explains how many cooking procedures work, and there are some fun kitchen activities/crafts included as well.
Happy Cooking!