a masturbator said:

I can only guess what my age was when I began playing with myself, but, if I
had to guess, I was about 5. My mother had my sister and I bathing together
until about then. It was also about that time that I began getting erections
and playing with myself alone in the bathroom. I loved the sensations at the
time and played with my penis a lot.
I was about 9 or 10 when I began masturbating the same way you did by self
discovery with my pj’s pulled down and rocking and grinding back and forth
in bed. I think I was hooked from the first time I actually came. I had no
idea what I was doing only that it felt so good. This form of masturbation
is called TMS and, I guess, leads to problems later on if one continues it
long enough. After being caught in the act several times by my mother I
forced myself to learn to do it with my hands in an effort to be more quiet
and not discovered as easily. My bed was just too noisy and she kept
catching me grinding away on my bed. The last time she walked on on me I was
about 14, it was in the middle of a hot summer day, I was naked and
squirting and very embarassed. It took a long time for me to learn how to do
it with my hands but I finally was able to. I masturbated every day, at
least once, from then on and continue to do so today.