When I was 3 years old, I took apart my mother's hair dryer. My dad came home from work and saw it in pieces on the floor. He yelled at me, and I put it back together, gave it back to my mother, and she continued using it.
I hear that story all the time because apparently taking apart and reassembling a hair dryer successfully is a remarkable feat for a 3 year old.
When I was your age, I was building anything I could out of anything I could get my hands on, so you're talking to someone who doesn't see age as anything but a number when it comes to building abilities.
That said, just be advised that building a tank is a big undertaking, especially if it's your first (I don't know if it is or not) DIY project. People on this board will tell you that I can build just about anything, but I will tell you that I don't build tanks, and it's simply because the stakes are very high. You're dealing with lots of expensive materials, and a LOT of property damage since a potential mistake is more likely to happen wihen you DIY.
So I'm not saying don't try... just saying be aware of what you're getting yourself into if you do try.
Does your $400 budget include the tank only or is it for the entire system? $400 should get you a tank in the size mentioned, but it will not buy an entire setup.
By the way, Perfecto makes a tank in those exact dimensions... 60x24x18 (don't do 12", the lack of front to back space will make aquascaping a nightmare). PetSmart sells it under their Top-Fin line for about $320.
