Don't recall there being a 12 year old on here. I was 12 when I first joined as a freshie. I have since moved over to the dark (salty) side. Aquarium1 and I are both 14.
I agree, there is no age limit to a tank, it all depends on the amount of time and money you are willing to dedicate to the tank. As mfd said, maturity is also a big factor so you can decide what is best for each fish, and not what YOU want.
I say go for it! Grab some base rock and do 75% base rock then get 25% live rock to help lower the cost of the rock. The base rock will eventually become live. I would keep pajama cardinals in your stock list as they don't add much to your bioload and are nice to have. Just get 3 as they like to be in odd numbered groups of 3 or more.
Hope this helps.