Well, lifespan should be measured in miles and not years. Obviously, a bike that is hanging up in the garage can stay in near mint condition for quite a long time compared to a bike that is getting ridden about 10,000 miles per year.

The following is about road bikes. This assumes are relatively decent quality bike (105 components or better). That being said, different parts of the bike wear out faster then others. Wear parts like tires, chains, cassettes and brake shoes need routine replacement. Other parts wear out but it takes a fair bit of mileage before they start going usually 20,000+ miles for things like deraillers, shifters, handlebars, seats, bottom brackets and so on.

The frame should last a very long time unless you have a severe crash.

Given decent maintenance most road bikes should get 20,000+ miles before things start to go and even then, it is probably economical to replace parts as they wear out rather then buy a new bike.

It is my observation, that very few bikes are retired because they "wore out". Even high mileage cyclists rarely ride a bike long enough to get much over 20,000 miles before they replace it. Generally people get the desire to get the latest and greatest bells and whistles or to get a more high performance bike in the usually mistaken notion that such a bike will make them a significantly faster rider.