ok, unless it is an autoflower the age is irrelevant because you can keep a cannabis plant in a vegetative state indefinately.you could take clones or use a supercropping method or simply top it.afterwards continue the vegetative stage until it has filled back out before changing to the flower cycle.
i agree with potter i would clone and only top if you are going to veg her long enough after topping for her to fill in and grow out.i would not do it if you are not going to veg her long enough afterwards for her to grow out and fill in.there would be no benefit at all,it would cost you in weight.top her and veg her back out and you have increased your yield......
Is topping only pinching out the centre? I would only do this if the plant isnt bushing out or getting too tall. Renember each end is a crown so pinching out the center can result in smaller buds too. But yes veg can go for way longer that 12 weeks. It also depends on the type of seed as some plants can double in size from end of veg to full cycle