It's no secret, the students age has a lot to do with it. Just a couple years back I was training a 12 year old and a 60 year old at the same time. I told the 60 year old to not get upset when the 12 year old soloed right away. The kid was ready to solo on day one but I made him wait until day two. The 60 year old took about two months to solo but then the rest of the year to get to the point of beginner. Now it's been two years and the kid is out of the hobby but the 62 year old is learning stunt flying, not just hacking around but nice smooth stunt flying. He can land a plane way better then I do pretty much every time.
Kids have no fear, they were born with a joy stick in there hands, it's not there money in that plane either, mom and dad shelled out for it and if it hits the ground mom and dad will buy a new one, that counts for a lot too.
Don't go by any time frame or you will set yourself up for depression, just stick with it, it will come to you if you just relax and enjoy the hobby.