I realized that I am still sleeping on the same bed that my grandparents bought for me (new) when I was in college (1988). How do you know when a bed needs to be replaced? There is no sagging, and no smells or stains. Is there some other factor I should be looking for?

one word: futon
best sleep ever

I keep hearing people say this, but every time I have slept on a futon, I was very uncomfortable.

Costco often has great prices on matresses. Only thing is you have to figure out how to get one home.

Go to Home Depot and rent their truck for $20.

I am the same. I slept on a futon for about a year and a half, and was increasingly uncomfortable. New bed end of February, it's been a happy girl I've been since then. First night of sleep was sheer bliss (and most of them afterward, too).

(Almost) Completely off topic, but if you want a really weird not of sleep, go in for a sleep study.

Basically, you go to a hospital (or clinic), allow them to attach little probes all over your body (in your scalp, around your eyes, air sensor around your nose, your legs, etc.) with wires hanging off them. They put you in your own little room with a tiny twin-sized bed (in said hospital/clinic - it's always so comforting to sleep in a strange place), and put a video camera on you so they can watch and record you all night.

And then they have the gall to tell you to "sleep normally."

They don't take credit and then I'd have to figure out what to do with my old mattress :(

Well, if you are buying from a mattress/bedding place, the usually will take your old mattress for free (with set up).

That's what dumpsters behind industrial parks and back roads in the forest are for.