You cannot literally "add" a child to your own United MileagePlus account, as every traveler must have their own individual account to earn and track miles. However, you can create a separate MileagePlus account for your child, regardless of their age, and manage it on their behalf. There is no minimum age to join, and miles earned by children do not expire. While the accounts are separate, you can link them in your "Profile" for easier booking, and you have the option to transfer miles from a child's account to yours (or vice versa), though United typically charges a fee for mileage transfers unless you are utilizing a specific "Mileage Pooling" feature. In early 2024, United introduced a Miles Pooling feature that allows up to five members—including family and friends—to combine their miles into a shared pool for free, which is the most effective way to "collect" a child's earned miles for a family trip. This ensures that every flight your child takes contributes to the family's overall travel goals without the logistical headache of fragmented balances.