Calamansi and other citrus fruits should be avoided till baby turns 1. Apparently, citrus fruits are allergy triggers, like egg whites, and should be avoided till baby is older. So citrus based juices like calamansi juice and orange juice, it's better to hold off. Consider giving baby apple juice instead, it's a safer juice alternative.
Aside from the allergy risk, citrus fruits contain too much acid that may not be very well digested by a baby's small, still-maturing tummy. I was instructed by ours not too!