you can apply for a sgc at any age you can not buy a gun until you are eighteen,you can have a gun on your license that is gifted to you,i would say that it would be ok for it to be kept at another house.the best thing to do is ask the fao when he/she does the first visit as I think you may be able to keep it at your home subject to normal security rules.also I believe at sixteen you are able to go shooting as long as you are with an adult and they do not need to have a cert of their own.as far as I recall this was the position when my grandson got his cert.