Caffeine, which is found in coffee, is a stimulant. Caffeine overdose can result in insomnia, agitation, stomach upset, headache, difficulty concentrating, and an increased heart rate. In younger children, these symptoms appear only after a small dose. Additionally, the most critical ages for bone development are childhood and adolescence. Excess caffeine can interfere with calcium absorption, preventing proper growth. It is best to wait until you are 18 before drinking real coffee on a daily basis (espresso, cappuccino, and latte).