In an interview it was stated that Shego was in her early 20s (this was after
Go Team Go but before
Stop Team Go), but they were totally fine changing their minds and the site that hosted the interview doesn't exist anymore so...
To be a substitute teacher does not require that you have your masters. It doesn't require much at all. My sister was a sub, my girlfriend is a sub, and neither of them have their masters. (My sister because she opted for farming over pursing higher levels of education, my girlfriend because panic attacks meant she never did her thesis.) Being a substitute teacher in
Stop Team Go means that by the time Kim is in senior year Shego is old enough to be a sub for high school students (varies state to state, but it basically means that she's at least 21 when Kim is 17/18.)
So that's our initial floor, she's got to be at least three years older than Kim to get the substitute teacher job.
Her degree in child development tells us only that she's had some college. It could be a two year degree, it could be a PhD (though I doubt it because then she'd probably say "my
doctorate in child development") and that's a big range. If it was a two year degree then she presumably had it by the age of 20 or 21. Given that "Shego never fails" in
Tick Tick Tick but Drakken hasn't taken over the world yet, she's presumably only just started working for him. If you place that when Kim was 15 then Shego could be as little as five years older than Kim (if she turned to evil immediately after getting her degree) or as much older as you want (if she waited a bit.) More on how long she's been evil in a footnote.*
So our floor's moved up to at least five years older than Kim, probably more. That's the end of the floor, by the way. There's nothing that
forces the age to go higher.
Then we get to ceiling. What can Shego not be older than?
And you know what? This is really hard to work out. Her dating Eric was presented as credible when Kim still thought that Eric was a real teenage boy. Various things like that point to them being a similar age. When Kim loses her memory (which shouldn't affect her ability to tell people's ages) Shego claimed to be only three grades ahead of Kim and while Ron was all, "No, old lady!" Kim thought it was believable. But nothing really nails it down.
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If I had to put this together into a coherent estimate, this is where I'd go with it:
Shego pursued a higher education in spite of being a superhero so either she got powers after she started pursing it or she didn't let being a superhero prevent her from pursuing it. Either way she was probably into college straight out of high school.
She doesn't call her degree a "doctorate" or "masters", in fact, from the way she describes it, it sounds (to me) like it's her only
degree. That would mean she wasn't a graduate student and thus her degree was only a two or four year degree. So she'd have gotten it at the age of 19/20 or 23/24 (depending on which type and when in the year her birthday lies.)
Then comes my biggest leap of all. She doesn't mention any teaching
experience so I don't think she used the degree. I'm taking that to mean that she turned evil not long after getting it.
Based on the age of the Wegos (see footnote) I don't think that was more than a year or so before she first shows up in the show.
So in her first appearance she's somewhere from 19 to 25, and by the end of the show she's ... well, when do you date
Tick Tick Tick? I think there's an argument to be made that it's the only episode set in Kim's Freshman year, but most people go with the fact that the opening credits say Kim is a sophomore (mind you, they say that when she's a Junior so ...) At the end she's from 22 to 29.
Lower than what everyone else says, potentially by a lot.
I'd try to keep her at the younger end of the range to stick with what the writers were trying to do in the first couple years of the series, and also to keep the implications that she's not much older than Kim true. I've put her as six years older than Kim in at least one fic of mine by assuming a two year degree and a quick turn to evil after that. That would be 21 at series start and 24 at series end.
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* We have some strong indication of the maximum time Shego could have been evil: the Wegos. Team Go included Shego and the Wegos at the same time. So Shego couldn't have quit when they were too young to fight crime. Now... what does that mean? I think that it's probably safe to assume that they weren't fighting crime at five, but what about ten? Jim, Tim, and Wade all had their first (intentional) field missions at 12, the Wegoes had superpowers and a bigger team so they could easily have done the same. For simplicity I'm going to say that ten is the cut off date. You can pick what makes sense to you and alter the math accordingly. Shego couldn't have quit when they were younger than ten.
Even if the Wegoes are significantly older than they look (and we have no reason to think they are) they're still teens at the time of
Stop Team Go. The oldest they can possibly be is 19, meaning that Shego couldn't have turned evil more than 9 years prior to the episode. That doesn't give us an age for Shego --siblings can be wildly different ages to the point that they get mistaken for parent and child when they're sister and brother instead-- but it does at least give us a range for how long Shego was evil.
Depending on how you place
Tick Tick Tick Shego has been evil for at least three years by
Stop Team Go but because she was still good when the Wegos were old enough to stop crime she couldn't have been evil for more than 9 years. Evil for 3-9 years at Season 4.
Honestly the Wegos are probably more like 14 years old. That would mean that Shego was evil between 3 and 4 years at the (second) end of the series. A pretty small range.
So she's been evil for almost no time, about a year, or (if you stretch things to the limit) five years when the series starts.