I would say 15-20 years is about the amount of time from when everything is new to everything is knackered.
Don't underestimate the effects of kids on a house. Smashing toys into the skirting boards, sticky hands on the walls, slamming the doors, spraying water all over the bathroom, spilling stuff on the floors ... it goes on and on.
Some things I have learnt :
Boilers : when you buy a new build, the boiler will be shit. That's because no one buys a house on the quality of the boiler, more like on how many bedrooms its got or whether it has a good garden. The builder will install the cheapest boiler they can get their hands on. Which means it will almost certainly be knackered in 15-20 years. So anyone moving into a 20 odd year old house will almost certainly need the original one replacing if it hasn't been done.
Fences : again missed this one when moving into my place. You want concrete posts and decent panels. A basic wooden post/horizontal overlap fence will only last 15-20 years. If you have concrete posts the panels can be replaced pretty easily and save you a ton of money in the process.
Doors : Again 15-20 years. Builders do not put high quality doors in new builds. Often people will overpaint the original doors where they are going rotten.
Electrics : generally modern wiring is pretty good, but in 20 years you will almost certainly have found the wiring standards have changed which means you will probably need a new consumer unit if you want to keep up to the latest standards.
Water/CH : radiators are normally furred up within 20 years (although if you have a magnetic trap you won't get this so much). Any hard water area you are going to get a lot of limescale deposits, which will probably ruin your aux immersion heater/hot water cylinder and also wear appliances like dishwashers and washing machines. Things like bar mixer showers will need replacing on this sort of timescale.
I could go on, and on. But its a bank holiday. And I have stuff to fix :(