A number of characteristics are stored in both the character IFFs and the neighbourhood IFF: age, gender, colour. The character IFF determines how the Sim looks. The neighbourhood IFF determines how the Sim acts, sounds and is treated by other Sims. (Ahem, so what is colour doing in the neighbourhood IFF? Latent racism?)
The easiest place to change the age is in the character IFF. Open the IFF in IFF Pencil or IFFsnooper. (Or a hex editor, but I once irreparably messed up an character IFF trying to change its age, so I'm not going into that here. And did we remember to back up our files before editing?) The screen caps showing the age flags are in Altering the bodystring. This IFF contains an age byte (9 or 27) and an adult/child label. In IFFsnooper I can only change the adult/child label, in IFF Pencil I can edit each line and so change both. That's the easy part done. The character will now look like a child and sound/act like an adult. Although it's wise not to rely on that; I had my child-woman Sim call a cab to a community lot, and once she was there, she became a complete child and so couldn't call a cab to return.
Now, the neighbourhood IFF. Not only is this much easier to hex-edit, it is the only way, because I don't know a fan utility that will change age and work with Unleashed. Fortunately, a fan has already analysed the neighbourhood IFF and published the result on her website, so I know where to look. In this IFF, I start with searching for the "user000[nn]" to be altered. From there, I count down the bytes. Every double number in the hex screen (whose string value is represented in an ASCII screen to the side, in UltraEdit - making things much easier, because pure hex is very easy to make mistakes in) is a byte. So, I count down: user header, five zero bytes, one number, seven zero bytes; total of 13. Next, a stats block of 34 bytes. Next, 54 zero bytes. Next, 20 interest bytes. Next - yee haw - "FF FF" for a child, or a career number for an adult. Be sure to change this to "00 00" ("unemployed"), or the career number if you want your Sim employed and have a list of career numbers handy. Actually, changing children to adults is easiest, because you can search for the first "FF FF" bytes to occur after "user000nn". What a shortcut, eh? To change an adult to a child, I have to count down all 121 bytes.
But that isn't the age, it's the career (which for children is "school" - and I've tried setting it to "00 00" to get round the hated military school, but that doesn't make a difference). The next two bytes are the performance level, or school grades for a child. The lower the number, the higher the grade; let's be wicked and set them to "00 00" for an A+.
After the four bytes that initially started with "FF FF" (so for an adult I'd have to count down 125 bytes in total) there is the age byte. It's followed by a zero byte, so the age is "1b 00" (27 in hexadecimal) for adults and "09 00" for children. Change "09 00" to "1b 00", save the file and the child is socially, as well as physically, adult. (To change an adult to a child, obviously, "1b 00" should become "09 00".)
(Due to the naming conventions, the Sim's bodystring will no longer be valid. So it's best to alter that before starting the game again, or the game itself will choose new skins, and the result might be bizarre.)
Intermediate values are possible. The age in the neighbourhood IFF determines how a child behaves and sounds, and this is where objects do their age checks, so if I filled in "11" (seventeen) the Sim would still be a child. However, it would sound like an adult. I've found that Simkids sound like adults from age twelve, so the highest age they can be given is eleven ("0B"). It is also possible to set the age to 9 in the user IFF's bodystring and 0B (eleven) in the neighbourhood IFF, but as the user IFF can be used to "refresh" the character's age, it's wise to set them both to the same value.
Changing a child to an adult
has an amusing side effect. Adults start out with zero skill points.
Children have randomly chosen skill points, but can't use them. The
child-turned-adult may have 5 Cooking Points and 9 Body points! A less
amusing side effect is that these points suddenly matter. In the child-Bison
family, any child who picked up the guitar was applauded, even Guy who had
only three Creative points. Once I'd turned them to adults, he was booed
every time he tried to play. And people say children are cruel?