An ISA investor must currently be an individual aged 18 or over (or at least 16 to invest in the cash component only).
Investors must be resident and ordinarily resident in the UK, or a non-resident Crown employee working overseas and subject to UK tax on their earnings.
An ISA can only be arranged on an individual basis (so it cannot be held jointly) and it cannot be held in a trust.
The Junior ISA became available on 1 November 2011. It is for children (i.e. under the age of 18) who live in the UK and do not already have a Child Trust Fund (CTF) account. Thus, broadly speaking Junior ISAs are available to any child who was born before 1 September 2002 or after 2 January 2011. A child can have one 'stocks and shares' and one cash Junior ISA to a total limit of £3,600 each tax year.
There is no restriction on who can pay into a Junior ISA, but the child cannot withdraw from the account until they are 18. If the funds are left in the account, the Junior ISA automatically becomes an ISA.