I disagree. First of all, what parts do you imagine would not be available after only 5-6 years? Derailleurs, brakes, headset, what? Surely you're not talking about tubes and tires, cables and chains. Parts availability is NOT going to be a problem.

Second, when you consider what used bikes sell for on Craigslist, you will end up with a much better bike if you are willing to buy one used and then replace/upgrade as needed. You don't say what your budget is but you will be able to afford a much better machine if you buy used, especially if all it needs is tires, cables, maybe new brake pads, chain, etc.

5-6 years old is practically a new model, and all you need to consider is the condition of the specific bike you're looking at. I would not consider age at all. Look at the configuration. How many speeds? Are those parts still available (cassette, derailleurs, shifters, etc.)? What kind of brakes (cantilever, V-Brakes, caliper, etc.)? Does the bike you're considering have any special, proprietary parts? Probably not.

For the same amount of money, would you rather have a brand new Chevy (with a warranty) or a 5-year-old Mercedes with 20k miles that only needs a set of tires, new windshield wipers, and an oil change? If you buy the Chevy, the warranty will run out in a year and then you'll still be driving a Chevy instead of a Mercedes.

The advantage of a new bike (as stated, from a LBS and not a big-box store) is they offer "free tune-ups" for some period of time. But you're going to learn how to work on your own bike, I assume. It's more trouble and takes more time to drag a bike to the store to adjust something than it is to do it yourself.

Buying used does require a bit more savvy, so you know what you're looking at and whether it's worth putting money into. On the other hand, buying new from a bike store is a no-brainer, you just go in and buy what you like.

So what IS your budget?