International driving licenses are accepted everywhere, however, they don't circumvent other restrictions, like age restrictions. No single national license is accepted everywhere.
Some countries driving licenses are accepted by others, sometimes reciprocally, but often not. The reason for this is due to the tests that the typical license holder went through and the restrictions their license applies.
For driving in the EU (and the UK) an EU or UK license is required if driving for more than a short visit (something in the order of months). These were standardised with each other and are accepted as one in the same.
For short visits to the UK, all licenses are accepted due to fairly recent relaxations in the regulations to aid with trade and tourism.
US licenses were not accepted until recently as they were considered too simple to obtain (the DMV queues and hassle don't count for testing your driving competency) and don't restrict access sufficiently (3.5 tonnes being the limit on a car license in the EU and the UK). Granted, applying for an international license doesn't test your driving competency, but it was an additional step that if you went through showed competency in another way - checking what you need to do to drive in another country before you get there.
The UK has a helpful form to find out your restrictions, though the information is incomplete: https://www.gov.uk/driving-nongb-licence
Other Restrictions and Requirements
Other restrictions, like age, can result in a normally acceptable license not being sufficient. It's my understanding that some states allow children to drive from as young as 14 - even with an international license, they would not be allowed to drive in the EU or the UK until they reach the age requirement by the driving country.
There are further exceptions to the above, but generally it's worth reading up on the destinations driving regulations and requirements. Little things like additional road signs, recovery equipment, and spares can also trip visitors up.
Some other legal requirements for EU countries I drive in that differ from the UK:
- spare bulbs
- high vis jackets per seat
- warning triangles
- stopping to help broken down vehicles
- lights on in tunnels
- tire chains (and in some countries, no tire chains)
- no radar detectors
First, it's worth noting that you can hire cars, vans, lorries, motorbikes, and other vehicles from most countries without showing your driving license. That does not mean you can drive them. It's also not to their benefit to try to restrict who can hire, unless the responsibility has been placed upon them by their insurers.
Insurance - insurance is null and void if you aren't allowed to drive the vehicle. In some countries, this is a criminal offence that could result in prison time. If you're involved in an accident and found not to have valid insurance, you could face prison time in many more countries.
In some countries, vehicles are confiscated from the drivers for lack of sufficient insurance, or driving without a valid license, along with on the spot fines. This could have knock-on fines from the hire companies.
Check your license for the destination country, get an international license if necessary, and learn the destinations rules and regulations.