The answer is some what less than straight forward, rubber wireing (which was phased out in the '60s) tended to last about 25 years without perishing) there were some rubber/pvc combinations and some inferier (to todays stuff) pvc in the '70s, but todays cable should last quite a bit more than 25 years if not overloaded. There are however some issues:
1) Regulations have changed in the last 25 years
2) Power demands have changed in the last 25 years (more kitchen appliances, etc, and less portable heaters about)
3) upto the early '80s 2.5mm² had a 1mm² (instead of 1.5mm) earth, this can cause problems with giving adequate protection to the cable, however it should be possible to choose the right device to do this (95% of times, it will be an MCB)
4) An installation might have been altered by many people with varying degrees of competancy, sometimes it could be easier to rip out and start again than check through it all
Chances are, if you installation is pretty tidy and dates back from say '78 (picking a year out the air) that some minor works will be needed, along with a consumer unit change, and the provision of some new circuits to bring it upto date for the 21st century
if you want to know whether a house requires re-wiring or not, the very best advice I can give you would be to engage the services of an electrician to carry out a PIR (periodic inspection) on the house. I'd say look for a full reg NICEIC member to do it *awaits nasty comments
*