Nobody should have issues with you lifting weights at 14, like most things just be sensible about it. Just bare in mind that you will still be growing till about 18 - 20, but you might stop at 6'4 you just never know. At that height and 18 stone you are looking at playing prop at a high level. Just don't sacrifice speed and mobility for size.
Why do you want to hit about 18 stone or there abouts?, that is very big. Have you found a club yet or is it schools?.
To give you an idea I found these figures for you. Dylan Hartley 6'1 16st, Paul Doran Jones 6'2 18st, Danny Herriott 6'1 18st, Ethan Waller 5'11 17st.
This is just a quick selection of Saints front row forwards as you can see at 18st your looking at prop weight.
The locks come in a little lighter and taller.
So really think about where you want to play and what size you need to be, just because your a lock now does not mean you won't be a prop etc later on. I would concentrate more on general fitness and rugby skills more than bulking up. Get good then get the size would be my advice.
If you do go for county trials you may well only be going for one position so you need to be good at it, and there you will meet other locks and players of your size. It makes it more interesting then as your trying to play better than your team mates
