At the age of 18, you can vote in general and local elections.
s.1 Representation of the People Act 2000

At the age of 18, you can become a Member of Parliament
s.17 Electoral Administration Act 2006

At the age of 18, you can become a local councilor or a mayor.
s.79(1) Local Government Act 1972 as substituted by s.17 Electoral Administration Act 2006

At the age of 18, you can get a tattoo.
s.1 Tattooing of Minors Act 1969

At the age of 21, if you are or were a ‘looked after’ child, your personal advisor pathway plan will usually end when you are 21 unless you are in further education or training.
s23C Children Act 1989, inserted by s.3 Children (Leaving Care) Act 2000

At the age of 21, if you were ‘looked after’ by the local authority between the ages of 16 and 18, the local authority continues to be under a duty to advise and befriend you. If you are in higher education the local authority has a duty to assist you financially up until the age of 24 if necessary.
s.24 Children Act 1989, inserted by s.4 Children (Leaving Care) Act 2000