A while ago, Lothere said this on the thread "Van decides to start being a good person and stop stealing other people's threads":

That's actually something I've always meant to play around with: setting people to incorrect life stages and incorrect genders in SimPE and see how that affects their in-game Sims.

Well, I'm not sure if you've tried playing around with genders and lifestages since then, Lothere, but you might be interested to know that I have managed to successfully change the gender of one of my Sim babies. Long story short, I had this woman give birth, and she had a son, but he ended up with a mouth that looks really bad on men. Like, really bad. So, since I don't have Uni and can't do plastic surgery, I toyed with the idea of killing the kid off, but then I decided to try changing him into a girl, since I know that the mouth looks okay on female sims. And it worked! Also, I tried it on a random adult Sim from Pleasantview -- and that worked too!

Here's what you do:

Open Sim PE, go through Neighborhood Browser as usual, and find the character you want. You'll see the "Treat as " buttons under the picture. Choose the opposite one.

THEN, open the character file and find gender. There should be a value that looks something like 0x0000001 or 0x0000002. Don't worry about the number of zeros, all you need to know is that 1 is female and 2 is male. Just change that last digit and... presto!

However, it does seem like one of those things that could prove to be fickle, so maybe only try it if it's an emergency, like if for plot purposes, you need whatever couple to have a boy and you try umpteen times and they just keep having girls. Or you could experiment on one of the Maxis Townies.

I hope this was somewhat helpful.