I have a friend in a local fish club that keeps only freshwater fish... but his daughter talked him into helping her set up a salt water tank... she's in the first grade (but not a member here)...
I have 2 salt tanks and I have to do just as many water changes on them as I do with my fresh. So I don't think that statement was entirely true. But anyhow I'm 18 so your younger then me.
I had salt for a long time, and while they do offer some really beautiful specimens, it is much more work than any freshwater in my opinion. Also, my freshies seem to be much more hardy, they can take changes in the water much better than salt. But yes 13 is quite young to have your own tank. My parents did the same for me when I was your age, had a 42 SW in my room and it was great; made me a fish guy for life