Practice makes perfect. According to Stephen LaBerge, eventually he could MILD easily after years of practice. As far as WILD is concerned, with practice you will be able to guide yourself easier into a dream rather than you could when you were a beginner.

I find no reason why a method would suddenly stop working for somebody. Just keep in mind tho, that with age, new concerns of the everyday life are getting on your way, so it is more likely someone will give up his LD efforts.