My father told me it would be better if it was retracted all of the time. I started when I was 14, longest I could go was 2 days until my over sensitive head was to much. He said the more I did it the less hyper sensitive it would be. I was off and on for the next couple of years. When I was 17 I fought threw it and stayed pulled back for nearly 1 month, now I am a pulled back full-time.
I am glad he got me started and I would recommend starting in your early teens.
Quite similar story here: my dad keeps his forekin back since his early teens, and when I was very young, he told me to do it too in order to keep it cleaner and healthier. The look of the bare head was "more mature" to me, and I used to think that all guys at some age started retraction, either naturally through the growing of the penis, or by keeping it pulled back all the time. By 14/15 I started trying to keep it back 24/7, and always covered after a little time, due to hypersensitivity. At my 17s I determined that from that year on I would keep the knob bare, and started little by little working on that. In the beginning it was hard, first I counted the days, then the months, and when I completed my first year uncovered, I had a sense of achievement, and realized I could keep doing it for whole life. Now the skin is retracted permanently, it has loosen up a lot, and the head got bigger and more flared. I'm glad my dad told me to do that. For the guys who want to achieve this, my advice is persistence, independently of the methods used.