Dude you have got nothing to worry about your a normal size for your age
I don't really think there is any age limit...As you said as long as it feels good " Enjoy " I think I must have started around the same age as you 9 or 10.
Simple answer No!! You would need to worry if you could not get hard at all....
I would go with washing before and after !! But hell what eva floats your boat. :grin:
At 13 your going to be an early bloomer or a late bloomer, im sure your ok and got nothing to worry abouts !! Just enjoy yourself and see what happens.
Just pure water is going to be the best thing the pain should die off after a short time...I guess she may not wanna go down for a while