My son never woke later than 5.30am from 10 months to 23 months, then we got a Groclock. We had a few rough mornings where he was getting to know we really did mean business with the clock, and within a few days he was going back to sleep at 5.30am and very quickly started sleeping til 7 or even later. My daughter, we never really had to introduce it as such because she has always shared a room with her brother and the clock has already been in use. But since 19 months she has known she has to wait for the sunshine before she can call us to get out of bed.
I remember telling friends at the time, if I knew how well it would work, I would have paid £200 for it, I was so tired. It's not for everyone and its not necessarily an easy fix and you do have to be completely consistent in how you use it, but it CAN work amazingly well. Unfortunately as everything there's no guarantees.