There's no simple answer to this. It depends which components you're talking about, which brand and even which product. If you want a merged home cinema/hifi or kit with a focus on computer/iPod-based audio then obviously you're looking at more recent kit for the functional changes. If you're looking at a cd-based (I can't comment on vinyl) stereo system then that opens your options more.
Then when deciding between second-hand and new I'd say you need to consider reliability (how likely it is to fail), support (what options are open to you if it does fail) and your subjective experience of the sound.
From a reliability perspective in a cd/amp/speakers system I'd probably be most concerned about 2nd hand cd players and how the transport's getting on, then power amps (does it need recapping?) and worry less about pre-amps and speakers. Considering support - is the manufacturer still in business? What's their attitude towards/ability to service older kit? Are the parts still available (e.g. Naim are great at servicing kit 20+ years old but there's no stock left of certain transports)?
Considering sound quality - I recently considered replacing my 10+yr old Densen cd player (cost about £1200 new) with the current entry-level Naim (cost about the same). No good reason, just a silly desire to spend money getting the newest/best. Anyway, I was pleased (and almost disappointed in one way!) to find I thought my Densen noticeably better.
Similarly, I've just bought a pair of 9 year old Ruark Crusader IIs, for £1k less than their replacements (the III believe it or not). I've not heard the III but have spoken to people who've listened to both and prefer the older model. Do I believe that loudspeaker technology has moved on so that my £500 would have been better spent on brand new speakers? Not if the models I listened to at dealers recently were representative.
From your original post it sounds like you might be happier buying new so that could be best for you (and my pre-amp was brand new this week!). Just bear in mind that newer doesn't necessarily mean better, but as a general rule you're not going to get product review mags or an industry arguing you might as well stick with your ten-year old kit!
Rob
Edit: forgetting which forum I was on for a moment I guess I need to make clear that my final comment wasn't a reference to conspiracy theories
. My point is that mags like WHF review new kit. They may make comparisons in passing to the preceding model or (as has happened recently) revisit older products to lop a star off, but it isn't their job to compare a new bit of kit with each permutation of 3, 5, 7 or 12 year old kit - that's what ears are for!