
Baby shrimp!! I got my first red cherry shrimp babies not long ago, and it's so exciting when it finally happens! I did some reading around, because I had no idea how to gauge their ages and growth rate either, but I went from first seeing a baby shrimp (like yours, not a tiny, invisible newborn, but a little bigger, and colouring up) then two months later, there was a berried female from those first babies. I knew they couldn't have hidden from me for four months even in dense planting. No way, so had to do some research.
It's going to be hard to tell for your specific tank, because growth rate apparently depends on both the tank temperature, and food availability. Lower temps, and their metabolism slows down and can take six months to to go from hatching to breeding age. Higher temps (my tank is at 79 F) means they can do that in two months. Of course, the other side of that coin is that they don't live as long if they grow that fast, it just speeds up their whole life cycle.
That's what I found for neocaridina, but I assume it holds true for crystals as well. Congrats on your baby(s)!