These are the general guidelines for England. There are many websites that go onto more detail, so look at those before you do something illegal:
If you want to enter a pub and buy yourself an alcoholic drink, you have to be 18 years of age or older.
If you wish to drink alcohol in public you must be 18 or over.
If you wish to buy alcohol at an off licence you must be 18 or over.
If you are under 18 you may enter an off licence provided you are with an adult, but you may not buy drink. An adult may not send you into any place to buy alcohol if you are under 18.
If you are 16 or 17, you may consume alcohol in a public place (such as a restaurant) if you are accompanied by an adult and eating.
At times, some pubs do allow younger children into their premises for food, often in designated ‘family rooms’. The cut off time may be 9pm or earlier. Check with the pub before you go in.
It is also allowed, though discouraged, for younger children to drink alcohol in private residences.
Proof of age
As with many countries, if you are lucky enough to look young you may be challenged to prove your age. We’d love to say this is nothing personal, but, of course it is. If you start to protest, you will not be served and may be asked to leave. The staff are just doing their job.
You need to provide some kind of legal, perhaps government issued, undoctored identity document that the staff will recognise and which shows your face and date of birth. A passport is ideal. Someone else’s say-so is not good enough.
Some properties, nightclubs for example, may operate stricter age rules eg
This will be clearly signposted. If you look young, you may be challenged.
If you are of a suitable age, why wouldn’t you be happy to undergo a simple check?