It depends on your kid! Some "easy babies" turn into challenging children, and some colicky infants become calm. A recent poll revealed that parents most commonly think kids are hardest to handle at age 8, when they want to be independent but are still prone to tantrums. Toddlers and teens can be tough, too—likely because these are both stages in which rapid growth in some parts of the brain outpaces development in areas that regulate emotion, say experts.