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Since you have lived apart for more than five years you can apply for a divorce based on that ground - no matter how old you are or whether or not you have children.
You can find the forms that you need here
(here if you are in Scotland)
You will need your marriage certificate (available from the local Registry Office to where you married) and the address of your ex.
If you do not know where your ex lives then you need to do your best to trace them, instructing an Enquiry Agent if necessary.
If you still cannot find your ex you can still go ahead but will need to ask the court for permission to go ahead without serving your ex using this form
and paying an extra fee of £50
Please ask if you need further details