Well. Technically, baby alpaca fiber is just the finest (thinnest) grade of alpaca fiber. It has nothing to do with age, only the “micron count”, or width, of the individual hairs.
A “micron” is a unit of measurement, actually a micrometer, used to characterize very small things (if you are a science nerd, you probably had to memorize the metric measurements, and you know that a micrometer is 1/10,000 of a centimeter). Generally, the smaller the number of microns, the softer the fiber. Alpaca fiber is sorted into grades according to the micron count, but the number of grades and their micron ranges differ from country to country and association to association.
1. Royal Baby Alpaca (<20 microns)
2. Baby Alpaca (20-22.9 microns)
3. Super Fine Alpaca (23-26.9 microns)
4. Adult (27-34 microns, sometimes broken into very fine and fine)
Baby alpaca yarn, while it does tend to come from younger animals, can be from an adult, if that adult has fine enough hair. Of course, there are other characteristics, besides the width of the hair, that determine softness. Sheep wool of the same micron count as alpaca wool will still feel coarser to the touch, because sheep hair has larger scales. But generally, anything over 30 microns doesn’t feel good next to your skin.