I got curious so i did some research and found this:

2151 - A shipping accident at Singapore International Spaceport exposes downwind communities to containers of dust-form element zero. [Kaidan exposed in utero to element zero] Alliance begins construction of Arcturus Station.

2152 - Roughly 30% of the children born in Singapore after element zero exposure suffer from cancerous growths. 70% seem normal.

2154 - Shepard is born; apparently was exposed in utero to element zero.

2156 - Some children of Singapore exhibit minor telekinetic abilities.

2158 - Humans learn potential of biotics. Original exposures now actively tracked. Roughly 10% of the initial exposed children show some level of biotic ability. Kaidan and Shepard not detected.

2160 - Conatix Ind. contracted to develop biotics program. BAaT training established on Jump Zero.

2163 - Earliest human-compatible biotic implants attempted. L1 implants yield minimal results due to late implantation. Accidental exposures too infrequent to contribute to research in a meaningful way. Second generation exposures "arranged" in a number of drive system failures over populated areas.

2166 - Turian "expert" brought in to Jump Zero to oversee biotic program.

2167 - L2 implants created. Kaidan among first to get the new implants. His potential increases dramatically but with discomfort. Others yield "unfortunate" results.

2168 - Shepard, 14 years old, received secondary exposure to element zero. Permanent biotic inclination manifests.

2169 - Kaidan, 17 years old, accidentally kills trainer after being provoked. Diplomatic incident forces closure of BAaT training. Kaidan refuses further training.

2170 - Biotic training assumed by military and select private R&D companies, renamed divisions of original Conatix Industries. L3 implants created.

2171 - Shepard officially detected as a biotic and fitted with L3 implants at 17.

2173 - Kaidan returns to military on his own terms. One of the few original biotics from the BAaT program that remains "stable." Refuses retrofit.

Apparently this is from the writer that wrote the Codex, it seems secondary exposure is one that happens after someone was exposed to it during pregnancy.

That interesting. Also I did not know that Kaiden was 3 years older then Shepard.

I honestly thought it was the opposite way around. I thought Shepard was older than Kaiden

I only knew that because i looked up sometime ago, but i used to think Shepard was older as well, i probably thought that because she's his superior, since they look about the same age to me at least the default versions of Shepard.

This only applies if Shepard was a biotic not if you choose him to be a Soldier, Engineer or infiltrator.

Shepard is a biotic no matter what, when you choose your class you're deciding whether Shepard will have trained as a biotic or not.

Not all biotics can use powers, only ones that chose to develop them, that is true even for Asari, it gets mentioned at least once in the games (in Illium i believe), that also explains why a Soldier or Infiltrator Shepard can have a biotic bonus power like Slam or Stasis.

Shepard could have been exposed to eezo by accident at any point during their childhood. Eezo is in everything from shuttle engines to eventual toothbrushes so use is clearly widespread despite the alleged scarcity of the substance. The entry states that 'permanent biotic inclination manifests'. That's probably why any Shepard could eventually use minor things like Warp ammo or a few other bonus options without other biotic initiators in-party. Only those that choose to train as a biotic class/specialization require being fitted with an amp.

Yesh, that's a good point, those 2 exposures are just the ones we know about, there could've been more.

Shepard was fitted with an amp tho, i think they're more so the person doesn't suffer any side effects rather than making it so the person able to use biotic abilities or maybe both.

Secondary exposure would be from having some form of contact with another person or thing directly exposed to Element Zero.

It's not all that difficult to get secondary exposure. Ellen Ryder made a career out of biotic implants. The constant element zero exposure made her ill . So if Shepard was exposed to element zero because his or her mom was. Secondary exposure could be a

laboratory accident. Perhaps even exposure in a mine. This could lead to biotics being developed. That's assuming the person doesn't develop cancer or some other terminal illness.

This raises a question though. If Shepard was a ordinary solider in Mass effect 1. Then is a vanguard or adept in mass effect 2. How does Shepard develop biotics? Shepard is on Cerberus' operating table for two years. Most of that time he or she is a blob of meat and tubes.