pre 1970, pretty well all, most of them don't have PVC cabling and many have unearthed lighting.
Anything more than about ten years old (and some new) may have insufficient sockets, especially in the kitchen and around the TV nest.
And will often have main earth bonds that are not up to scratch. Both these can be modernised quite easily without a rewire.
Between them, depends on the house. A qualified electrician can motly tell with 30 seconds of walking through your front door and looking at the CU, but will need to carry out further inspection and test to see what else is wrong.
Have a look at your consumer unit and see how many circuits are not RCD protected. The rules have got increasingly strict over time. I'd say, as a minimum, your electric shower, all your sockets, your boiler and immersion heater should be, and anything outdoors including sheds, fountains, christmas trees and outdoor lighting. If your CU is (sucks teeth) fifteen years old or younger, and a major brand, you can probably have it upgraded with RCBOs which replace the existing MCBs and incorporate an RCD for each circuit. This is actually a superior method.
If you do need a new CU, get a good big one with plenty of spare ways for future expansion. A large empty metal box costs little more (sometimes less) than a small empty metal box. All new domestic CUs are now steel as it is considered non-flammable. If your CU is under the wooden stairs or some other flammable thing, that's another factor guiding you towards replacement. Have the new one spaced away from the ceiling or stairs, and the ceiling holes firestopped, as flames can go up the holes for cables into the void above the ceiling, where the wooden joists and lots of dry dust are found. You can, instead, have a steel cupboard put round your old CU, but I doubt anybody ever does that.
You need a qualified electrician who is a member of one of the Self-Certification schemes, and preferably an Approved Contractor rather than just a Domestic Installer, which is a lower grade. NICEIC is an example of such a scheme, there are quite a few.