The law does not set down the minimum age at which you can be left alone; the idea is that parents and carers will know best whether it is safe to leave a particular child alone in a particular situation, and for how long. (The law does punish parents who put children at risk of danger or suffering through neglect, but this is rare.)
So, this is something for you to discuss with your parents or carers; so that whenever you are left alone you feel happy, safe and confident, and know what to do.
There are recommended age limits. The NSPCC (National Society for the Prevention of Cruelty to Children, a child protection charity) recommends that children under the age of 12 should only be left alone for a short time as they may not be mature enough to cope with an emergency; and that children under the age of 16 should not be left alone overnight. You can find the helpful guide from the NSPCC here.