Miranda's given age is 35, while Shepard is around 30. That being said it's been pointed out that Miranda's age has her predating the first recorded human biotics by a few years.
Given that they also list Zaeed as being 40, I'm willing to doubt the accuracy of ME ages, but I do think Miranda still works best in her early to mid thirties.
Actually, I am pretty sure (without checking a wiki) that Shepard is 31 in ME1. So in ME2, Shepard is 33. Which gives Miranda two years on Shepard. Kaidan is 32...33?
Checking the wiki: ME1 takes place in 2183. Shepard is born 2154, Kaidan is born 2151, and Miranda is 2150. Respectively, 29 (whoops!), 32, and 33 (so by ME2, it would be 31, 34, and 35).
From what I gathered from conversations with Kaidan in ME1, the Singapore Incident wasn't the first batch of human biotics, but I think it was when they tied the eezo exposure in utero to biotic potential. It was just the only incident large enough that warranted a place on the official Time Line. We also learn that Cerberus was responsible for that exposure during LotSB, among others. So, considering her father's ties with Cerberus, it is no surprise they were able to help him engineer a biotic way before it was happening naturally. Only the first he kept, remember? Trial and error and then they probably used that to expose other humans to increase "humanity's potential".