In case anyone is curious, I figured out the timeline about biotic Shepard. Shepard is born in 2154, and then gets secondary EEZO exposure in 2168 at the age of 14 which makes Shepard permanently manifest biotic abilities. Shepard then goes another 3 years as a biotic before finally being officially detected as being a biotic, and only then gets the surgery for an implant at 17. Shepard then joins the Alliance 1 year later. This is unusual because most biotics get detected early and get their implants around the age of puberty, whereas Shepard was practically an adult already before getting one.
http://masseffect.wi...m/wiki/Timeline
Also, if you're wondering about the secondary exposure being needed, Kaidan mentions that a few of the people who show biotic inclinations as children actually lose their biotic abilities as they grow older, and eventually lose them forever*. I think that was going to happen to Shepard, until the secondary exposure happened which made the biotic inclinations there to stay for good.
Neutral (Shepard) – What’s the survival rate? Seems like you beat the odds. How many didn’t make it?
Kaidan: Out of a hundred? Maybe sixty have no effect. Thirty suffer “adverse effects.” Little things like brain cancer. The other ten show enough ability to augment with implants. Not always permanent, though. Not like the cancer. Next thing you know, you’re out on Jump Zero. How’s a kid supposed to deal with that? A station at the edge of human space?