UPDATE: Science marches on!

Urist DaVinci has found a cure. Putnam and Kubrik Amudnil each developed a patch script for DF Hack.

A new version of PeridexisErrant's Lazy Newb Pack has been released that incorporates Kubrik's script.

That said, if you want all of your (vanilla) creatures to be born adult sized, this is the patch for you.

Patch for Bug 0006334: Creatures, including dwarves, born in the fortress do not grow to full adult size.

See this thread.

The bug is preventing creatures born in the fortress from growing, limiting them to their birth size. This reduces both combat effectiveness and butchering results.

This patch contains the Object folder with patched vanilla raws, and a .dfmod file created using DF Mod Manager 0.7.

Mod replaces all instances of the [BODY_SIZE] tag for which there are multiple stages of growth. The first stage body size is replaced with that of the last stage, and subsequent stages are deleted. For example, in creature_standard.txt, under [CREATURE:DWARF],

[BODY_SIZE:0:0:3000]
[BODY_SIZE:1:168:15000]
[BODY_SIZE:12:0:60000]

is replaced with

[BODY_SIZE:0:0:60000]

This change forces all creatures to be born full size, restoring fortress born Dwarves and War Animals to their proper combat prowess, and livestock to their proper food yield.

Note:
- Does not affect creatures born in the fortress prior to applying the mod.
- Bug does not appear to affect World Gen. I recommend that you apply the mod after World Gen, but before embarking for the first time.
- I do mean ALL creatures. Dragons and other Mega and Semi-Mega beasts no longer need many years to reach full size. Proceed at your own risk.