It's not quite so simple. Although the age of majority is 18 according to the Family Law Reform Act 1969, that is primarily relevant to financial contracts (debt), criminal sentencing, and tobacco. Drinking, you can drink in private from sixteen, although you can only purchase once at eighteen. You can start a company at sixteen. You bear primary authority over your medical records from sixteen, and your caregivers cannot access them without your consent. You can vote in some elections from sixteen. But you can't adopt until 21.

The TLDR is that in the UK, age limits are quite case-by-case, and tend to range from 16 to 21, although in more niche cases can range from 11 (the age you can open a bank account) to 30 (the age of suffrage for women in the 1918 Representation of the people act).

Wanted to mention that 16 year olds could buy cigarettes until 2007, went to check the year, found that more interestingly:

You can smoke cigarettes at any age, but you are not allowed to buy them until you are 18.

If you are caught smoking by a uniformed police officer or park keeper in a public place when under 16, he or she can take away your tobacco and cigarette papers.

s.7 Children and Young Persons Act 1933

https://lawstuff.org.uk/at-what-age-can-i/buy-and-consume/