Age 17 (borrowing age as set out in law) without certificate (not talking about club terms, talking about a firearm on someone elses certificate) and only under the exemption under the
1988 firearms amendment, which was further amended and very much expanded upon and simplified in wording by the
2017 policing and crime act, which makes it very consise and clear. The person must use the firearm within sight/earshot I.e.
supervised and
must obey the same certificate conditions. That person must also have
written authority to be able to lend the firearms on that land, if they are not the owner of the land or owner of the rights. The lender must also be aged 18 or older.
As it states the borrower must follow the conditions of the lenders certificate, if the conditions state any lawful quarry and zeroing and target, that person can do indeed just that under supervision. Needless to say, the person borrowing said firearms from said lender are also required to permission to shoot that land, as you are, if you get that authority from the land owner and are not the land owner or rights owner yourself.
Law below
An Act to make provision for collaboration between the emergency services; to make provision about the handling of police complaints and other matters relating to police conduct and to make further provision about the Independent Police Complaints Commission; to make provision for...
www.legislation.gov.uk
14 is the age if the person has their own certificate with conditions that allow shooting quarry in the field of course. At that age they will most likely have a mentor condition and be supervised anyway, even if the firearm is on their own certificate or shared ownership with the mentor. They cannot buy their own firearms or ammunition until 18, it must be gifted.
Always follow direct law extracts from official government sources, not heresy on social media or magazine articles.
Hope that answers your question.
Screenshot of law below.
For more information if confused, talk to BASC/ your firearms dept but the 2017 rehash makes it very clear. Be sure to keep a paper trail of wording provided to you I.e. in email form if you do talk to someone about it.