In terms of puberty I started earlier than most, but my muscle tone has remained constant. That is, I grew facial/pubic hair, but I remained a weakling for quite a long time.
Although I am trying to start to become more fit. I asked about it on this site called 4chan, and they recommended me this site called "the Starting Strength Wiki."
I can't post any links yet, so look it up.
I've been following it for a few weeks now, and I must say that I have been improving quite a bit.
Also, look up "scooby1961" on YouTube for a reliable video source on fitness.