Well, that’s a coincidence!.. I’ve just finished knitting the exact same sweater, but mine is 0-3 months size and has short sleeves, striped but differently. I used the same basic pattern as you, with some advisory emails from someone on Ravelry who also made one with short sleeves. My sleeves stopped, with a couple of garter ridges, just about where your’s are now.
But that doesn’t help you with your question about sleeve length. However I knit lots and lots of baby clothes, frequently Sirdar patterns, and the sleeve lengths tend to be:
0-3 months = 14 or 15 cm
3-6 months = 16 or 17 cm
6-12 months = 19 or 20 cm
I don’t think there’s any rule about sleeve to body length ratio, as body length can vary according to the style of the garment. For instance, a bolero style cardigan will have a shorter body length than a hoodie, yet the sleeve length will be the same for each.
I hope this is of some help. You could also look at the measurement tables in a few baby patterns and see what sleeve length they quote for different ages.