I have a Kelty 4 man dome trail tent that I purchased in 2004.  It's been a pretty solid tent with 40 or so outings over the years.  Took it camping last night and it leaked through the fly.  It rained pretty good but I would swear it was coming through the fly and then dripping on the screen and us below.  

Are there any treatments that can be done to the tent fly to make it work better or is it time for a new tent?

From a long term perspective how long will a tent last outdoors before UV and weathering do it in?  

Realizing that each tent quality and such vary just give me your personal experience or ballpark estimates.

First off, are you sure the water wasn't coming from condensation build-up on the inside of the rain fly?  If it's cold out, in high humidity conditions the moisture from your breath can make it damp underneath. When you stake out your rain fly, make sure it's tight and not touching the main body of the tent fabric anywhere as this will create a wicking effect with the condensation.

Yes, there are products to restore the waterproofness to your tent fly.
Check out an REI store for a DWR (Durable Water Repellent) product or contact Kelty and see what they recommend.

UV and storing your tent damp can both contribute to a breakdown of the waterproof barrier.
Here's a link for some helpful tips on gear repair/maintenance.
http://www.backpacker.com/gear/backpacks/make-your-gear-last-forever/

10 years is a good run on your gear. It's probably got plenty of life left, though.
However, materials technology has improved greatly over the last decade and you might want to think about upgrading.

I'm almost positive it wasn't condensation because it occurred right during the first rain band.  We had only set up the tent and hopped inside to eat our dinner when the downpour started.  Stopped an hour later.  From what I could tell the water was coming out from under the seems where the tent poles (think bow) were stretching the rain fly the most.  There was also a seem there.  The interior tent did not touch the rain fly.  

Think I'll look into the dwr finish and your other recommendations.  Thx

Definitely try a DWR. Typically that's either a spray on or a brush on. You can also get a thicker seam sealer that is made specifically to dry in the needle holes from sewing. Or get new seam tape to put over the seams. Its also important that the fly be good and taut. If it is sagging anywhere that the water can pool, then it will start to leak from the standing water pressure at some point. Or when condensation does build up on the underside then the molecules will also want to pull other molecules through as water likes water. And, in my experience, they will all leak some like that at some point if it just rains all night or really hard for any amount of time. Those fabrics aren't impermeable. If I have a lightweight tarp of some type with me, say if we brought one to cook or hang out under in case we are up when it is raining, then I will sometimes set it up over the tent as a second barrier if I know rain is expected. It could (will) start leaking at some point too but it buys time if you're gonna be sleeping in the rain all night.

That's a good point.  I like the idea of having a second fly to cook under/ hang out under/ extra layer to shield tent.  

I did have a couple spots where the water made small 1-2" wide pools.  The second band of rain lasted almost 10 hours.  Tent was set up on a slight slope.  Very slight.  But it caused the waster to sit in one or two spots.  Every minute or so I would hear the water run off of the spot because it filled up and ran over.  Haven't camped in stuff like that since I was in boy scouts twenty years ago.  

I could see water seeping through the seams at the sides and door where the rain fly didn't cover.  

Read your post again:  the rain fly was as taught as it could be.  This tent has good attachment points.  And I had all of the pullouts out at optimal angles.  The only place the fly contacted the tent was the poles.  It got close at the bottom 9-12 inches but the poles gave it some space.  

I'm thinking a Kelty Noah tarp or something would be nice as a spare.  Then again I'd really like to try hammock camping!

Call Kelty and ask for a replacement fly.
They might just send you a new tent.
Fwiw Sierra Outpost has tents marked way down right now.
I purchased a two man Kelty Salida for $101 last week.

I have an old Timberline tent from 1997's and it still works well.  Wife and I usually use it 2-4 times a year.  I make sure it's completely dry and then keep it in my spare room closet where it's dark and cool.  The fly on my tent did leak once about 15 years ago so I applied waterproofing spray on that came in a can.  It worked for couple years and I had to spray it again.  About 2 years ago I sprayed on watershield spray on stuff and it worked good too.  A well made tent will last a long time if you take care of it.

sounds like you need to reseal the seams. It has been used a good bit. That and a spray on DWR should do the trick.
Also take a look at the underside to make sure it is not sticky or peeling.