I'm going through mattress trauma, so I had to weigh in. We had our first mattress approximately 10 years, and then, when I was pregnant I found I was having hip pain from sleeping on one side so much. We bought another, softer bed. Then I ended up co-sleeping, so ended up on the old bed, and he ended up on the newer bed. We FINALLY put back the new bed, and I got a few months on it, and my husband accepted a hand-me down king (I really didn't want him to do it.) We made it 10 days before we got delivery on another new bed, gave away the 13 year old Queen, and moved the 4 year old Queen downstairs as a guest bed (on my beautiful sleigh bed). The new bed was defective, so tomorrow we get our third attempt by the company to give us a bed that doesn't suck.
Meanwhile, my uber-comfortable queen lives downstairs, without me.
Sorry, that was more a rant than informative. But I have to say, I would even prefer the 13 year old queen to what we have now. ARGH!
If you are still comfortable, don't waste the money on a new bed. If you are starting to be uncomfortable, go lie on some new beds and see if you can find something you like in your price range. - there advice, instead of just whining about MY bed situation (which is really bad). I think I'll go sleep downstairs, now that I think about it.
My daughter has a hand-me-down twin from my first daughter (who used it for around 5 years total). When she's a little older we'll eventually get her another twin. Her room might hold a full, but it would be tight. But fulls are nice if they want a sleep-over. Of course, you can have a trundle for that, or something like that.