Hi Jules,
I started the same way you are now. Silver threads running hear and there (I was never lucky enough to have a streak). There is no stopping your grey now - it will come in more and more.
If you color your hair, henna or chemical, you will have roots of grey. More and more pronounced as time passes. This is one of the reasons I chose not to color. I just can't keep up with that sort of maintenance.
About the shine... At the root of every hair there is an oil gland. Oil glands respond to hormones. Think of what happened when you became a teenager. I don't know how old you, are but I'm 54 and have gone through menopause (surgically). I just don't produce the oil I did when I was younger.
I now put olive oil in my conditioner to help with the shine and snarls (I use a non-oiled conditioner on my scalp - otherwise I have to wash in 3 days). My hair loves it - it is so soft! Thanks to who ever posted that suggestion. It works great!!
Going grey is not an easy decision. I think it ages me a bit. I'm glad you are thinking about it first. Let us know.

Christmas 08

April 09
Close up of my gray hair.
My new mantra: 2009 for hair that's divine!
Oil on ends just before combing to help with snarls, 2x a day.
Shampoo every 5 days
Experimenting with ACV, coconut oil, and honey.
Started measuring August 06, at 36.5 inches
1cCii ~ Natural Silver
I am 53 year old and I've been married for 34 years!!
July 1st 07 - Reached 40 inches
July 14 07 - Cut back to 36.5
August 3rd 08 - Reached 40 inches again.
January 1 09 Reached 43 and classic achieved! Cut for a better hemline.
April thru June 40 still. sigh...
36.5/40/Classic
Faithful to barbers - they actually trim 1/8th of an inch!!