My son is 12 this month, he’s been knocking about the kit since day one and seeing deer and birds hung up here and there etc.
He started coming out high seating probably at 7 or 8. My advice is bring food, hot chocolate and don’t make the sessions long. Don’t get frustrated with noises etc, and make sure you get something shot before too many trips. You have to realise that they are probably there to be with you, rather than specifically because they want to see a deer shot. They are actually doing you a favour being wit you. Although it seldom feels like it

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He now likes to come out with a mate, we have a 3 seater high seat that is ideal. As long as there deer about even if not shootable and just seen in the thermal, they remain engaged. Sometimes the phone has to come out on low light setting for a short hunting game.
He came on the hill with me and my brother for the first time this November in terrible conditions. He was in his element and could have turned round and walked back up when we finished despite being wet through and 30m visibility down to fog in some places.
But you know what - even though he shoots an air rifle pretty well and often asks to come out, he prefers rugby, cricket, surfing so my time is spent facilitating that more than trying to make him like my hobby. Best I can hope is that he grows up as an advocate of the sport and knows he can access it though me, and I’ll allow him to find his own way rather than expecting him to want to do more shooting than anything just because I do.
Final note on a long post, don’t feel you have to rush to letting them have a “proper” gun. If things go wrong their and your life is blighted forever. You hear about it and despite all safety precautions the potential hazard if something goes wrong is massive.