The best way to learn AIX would be to obtain an account on a machine that's running it. Really, part of what sets AIX apart from other unices is that it's designed for high-end systems (with lots of processors, fancy virtualization capabilities and so on). You won't learn as much by running it in a virtual machine.
If you really want to run AIX on your laptop, you'll have to get an old
PS/2 version that runs on an x86 CPU. I don't know if AIX will run on VirtualBox's emulated hardware (PS/2 is peculiar, it's the same problem as running OSX in a VM), but there are hints that it might (user claiming to run an AIX guest). It seems that AIX can run in Virtual PC.
Qemu can emulate PowerPC processors, so it might be able to run a recent, PowerPC version of AIX. But don't get your hopes up.
In summary, getting AIX in a VM would be costly (it's not free software), difficult, and not very useful. Try and get an account on some big iron, or get a second-hand system (if you can afford it).