Ethsharites think of life in six-year blocks. Under age six is an infant, of whom nothing much is expected; six to twelve is a child, who ought to learn reading, writing, and manners, but still isn't entirely responsible. Twelve to eighteen is an apprentice, the years when a person should be learning a trade or otherwise preparing for adulthood. Eighteen to twenty-four is a journeyman, when a person should be finding a spouse, settling down, establishing himself in his business, etc. Twenty-four to thirty is a distinct stage I don't have a name for, as there's no equivalent in English, when a person is established but not yet a Master, a parent raising young children. And over thirty is a full adult.
Children under age twelve wear simple knee-length T-cut tunics--sleeve length depends on climate, but they're generally loose garments, unconfining. No pockets; anything to be carried is hung on one's belt.
At age twelve, a boy starts wearing breeches and stops lengthening the tunic as he grows; by adulthood, men's tunics are therefore typically hip-length, though someone who hit a really major adolescent growth-spurt after age twelve might wear his only to the waist or so.
Men wear either breeches or a kilt; a kilt is considered a sign of virility, and is standard garb for soldiers, sailers, and bridegrooms. A boy under age eighteen who wears a kilt without being one of those three is bragging, and will be teased for it. Kilts are generally solid colors, rather than plaids; soldiers wear red, sailors wear blue, and other men whatever they please.
Women start wearing skirts either at age twelve or after their first period--family custom as to which. Skirts are ankle-length. As with men, the tunic stays the same length it was at age twelve, which means it comes to the upper or middle thigh, generally; it's worn over the skirt, never tucked into the waistband.
None of these rules apply to the nobility, the very wealthy, magicians, or people dressing up for special occasions; in all those cases, fashions change constantly and just about anything goes. Gowns, robes, dresses, and blouses are common.