Really, there aren't clear averages for age groups. And really, unless your doctor has told you there's a problem or that your penis is unusual, chances are you're as average as anyone else.
In terms of the G-spot, because it's internal, diagrams don't help much. You can certainly look for yourself, with something like this one, for instance -- http://www.scarleteen.com/body/female_anatomy_5.html -- you can see it about halfway up the vaginal canal.
It's easier to find by touch, really. Even with a latex glove on, if your fingers are inside a woman's vaginal canal, curved upward, towards her belly rather than back, you can feel the slightly textured spongy tissue pretty easily.
It stands to mention, however, that the g-spot isn't a magic button, and finding it isn't really like finding the Golden Fleece. Like anything else, some women like g-spot stimulus a whole lot, others are left lukewarm, and very often, g-spot stimulus all by itself isn't going to work any special magic.