I have a Pflueger Trion that's 6 years old that has an extra spool which I just found. It has Pline Flouro Clear 15 pound test on it. Can I still use this or should I disgaurd and respool?

Id replace half of it. Leave the rest as backing,

You shouldn't have any problem as long as the line wasn't exposed to high heat.  My largest fish ever was caught on 8# Trilene XL that was several years old.

Heat, water, and light can all take their toll on line.  I can tell you a dozen stories of my dad losing fish on dry rotted line.  For how cheap mono is, it's worth the $10 just to get a new spool. 

Like the others said.  

Why risk it?

Line is cheap and the only  thing between  you and the fish.

Its better to be safe than sorry.

Why would you want to save the line?

Won't hurt to cast it to see how it works, but IMHO, it's too high pound test for good performance on a spinning reel anyway.  I would replace it with a name brand 15 pound braid, use a leader if you want, or not, and you'll have much better performance.  If new to braid, do some research on knots.

For what it'll cost to replace, and considering it's the ONLY thing connecting bait to reel...  Replace it.  Not worth the risk of losing a fish, a bait you like or even having a doubt IMO.  

Count me in as in favor of replacing it. Now if you told me you had 6 year line sitting around on the spool it was it came from the manufacturer on, you would be fine to use it. Since it has been sitting on the reel all this time on a spool that is much smaller in diameter it is probably similar in look to a slinky. I would just respool with fresh line, as the line that is on there is cheap anyway.

If I were closer, I'd give you some line. I lost more money in tree snagged lures by the end of January this year then I will spend on line. 

15lb is too heavy for a spinning reel anyways, I'd replace it. 

Why take the chance on losing the fish of a lifetime. I would strip the entire spool. Doesn't cost much and doesn't take more than a few minutes spool. No brainer to me- replace it all.

Thanks everyone, will be replacing the line tomorrow.