The minimum age for the granting of a firearms certificate is 14 years old.
You must be able to prove to the police you have a ‘good reason’ for acquiring a hunting rifle.
Evidence of good reason is usually provided by a piece of land over which you have permission to stalk, or a firm booking through an estate or sporting agent.
It would be a good idea if you found a mentor in the form of a local person, someone who would take you out stalking and show you the ropes. You would also enjoy and benefit from an introductory stalking course, such as the DSC1.
This is not compulsory, however, and the police cannot insist that you do this. If you have no experience of firearms use, the police may put a condition on your certificate that requires you to be supervised by an expert stalker. The length of this supervision will be a matter for discussion and negotiation with the licensing department.
You can gain valuable experience by joining a target rifle club. Hunting rabbits and small game with an air rifle is excellent grounding for deerstalking, as it is a combination of marksmanship skills and fieldcraft.