All criminal records are for life.
Most convictions become 'spent' (for most purposes) after a certain period of time. The length of that period is determined by the sentence imposed by the court. For people under 18 on the date of conviction, the period is usually only half of that which applies to adult offenders.
(See the link, below, for details of the relevant periods).
Once a conviction is spent, it does not have to be declared when applying for such things as insurance and for most types of employment.
However, when applying for jobs involved with the administration of justice (police, court service, prison service, etc) or working with, or alongside, children or vulnerable adults (teaching, school caretakers, nurses, hospital porters, etc) convictions never become spent and must always be declared. For such types of employment, potential employers can insist upon a CRB check and all offences will show up (irrespective of the age of the offender at the time of conviction or whether the conviction would normally be regarded as spent).
I notice that you've also posted this question in the Travel section. Maybe that was just an error but, if your question relates to a visa application, you should be aware that the Rehabilitation of Offenders Act doesn't apply to other countries. (i.e. other countries don't recognise convictions as being 'spent'). You'll always have to declare your conviction, if asked to do so, when applying for visas to enter other countries.
http://www.lawontheweb.co.uk/rehabact.htm
Chris