As lots of people have said, the RF noise floor in the area will increase over time as more "wireless" (especially 2.4GHz, and some 5.8GHz) gear is deployed (especially in the 2.4GHz ISM band, not just "wireless" computer networking, but lots of other stuff). Since the ability to recognise/decode your desired signal is significantly affected by signal to noise ratio, over time the RF environment gets worse.
New equipment generally comes with more sensitive receivers, which can work with a lower signal to noise ratio (over the couple of years I was following it closely, the improvement was something like half an order of magnitude less signal required -- relative to the same noise floor). Newer equipment often comes with new standards too, which also help improve reception (either changing to a less crowded band, eg 802.11A in 5.8 GHz instead of the crowded 2.4 GHz; or by better antennas that "localise" the signal -- eg, 802.11N deployed with multiple antennas and complex decoders to mix what it learns from each together for a better overall decode). So, in general, newly designed equipment is going to work better in the same environment than older equipment using the same standards as everyone else nearby.
It's also technically possible that there is increased resistance in, eg, the connectors between the transmitter/receiver and the radio antenna. This is mostly a risk in outdoor installations (where weather can get at it, particularly water -- usually in the form of water in the air, rather than rain; if you're getting direct rain on bare connectors you've already lost!). Outdoor installations near the sea are particularly at risk. Good weather sealing is critical in out door installations for this reason.
Usually one of the best things you can do to improve your reception and that of your neighbors (ie, encourage them to do it too) is to position your wireless APs as close as possible to where you want to use them (and if you have a large house, use more than one rather than trying to find a super "covers the whole house at once" solution; if they're got the same access credentials and are on the same LAN, modern computers should switch around as needed). RF power drops off rapidly the further you are away from the source, so getting closer to the source gives you a much better connection for the same amount of power. If you have the option to change antennas (at least older consumer wireless gear had that option, as does a lot of business gear), then picking an antenna design that focuses the power you're allowed to use into the area you want to use it will also help significantly (for the same reason). Eg, using a sector antenna to just focus everything into one corner. It'll help if your neighbours do that too. (And if you're talking to them, and they seem to have a clue, try to arrange not to be on overlapping channels.)