? Tube Replacement Question ?

As a novice in the world of vacuum tubes, I have a couple of basic questions. I have a Audio Research SP16 tube preamp I purchased new in January 2008 & use on a daily basis. The tubes have never been replaced.

(1) When is it prudent to replace the tubes?

(2) Will sound quality deteriorate with age?

(3) Are there advantages of replacing the tubes with aftermarket alternatives vs. OEM tubes?

Thank you kindly for your help.
I recently owned the power amp, VS55 that matches your preamp. When I bought it I emailed Jim McShane and was very pleased with his prompt reply and his knowledge. I took him up on one of his suggestions and was very pleased with the results. My trade is law. I do tinker, but I also respect expertise. Especially when the one providing it is willing to explain and offer alternatives.
Rodman99999 makes the point that replacing all the tubes with certified low-noise tubes is "well worth the cost". Since I do not use the phono stage, is there an obvious difference in sonic quality will all low-noise tubes?
"...the quiet ones were where they needed to be (phono input, Post RIAA amp, and line input).The not so quiet ones went where they didn't cause any problems (cathode followers and intermediate gain stages. By doing this we could use 95% of the tubes. You don't need a quiet tube everywhere and that is why RAM Tubes come in 4 grades." RM
The gain is not as high, in the other stages(compared to phono related) of your preamp, but noisy/microphonic tubes will still be audible. Upscale's graded tubes are tested for triode balance, as well as noise/microphony, which will also improve channel balance/imaging, in the SP-16. Everything hinges on how much you value performance.

With my sp16, I hear a tad more hiss/background noise only in the phono stage at higher volumes with nothing playing using the Groove Tube Gold 12AX7s compared to the original "low noise" tubes that came in the unit for ARC. Not an issue for me but for those who cannot handle any extra noise at all in their phono stage ever, it might be.