Having a baby when you're in your late teens or early twenties is a completely different experience than having one when you're in your 30s. A lot of people have a kid when they're really young and wait a long while before having any more kids because they were frankly unprepared for their first kid. The difference between parenting as a clueless kid whose brain hasn't fully developed and a full-tilt adult is incredibly palpable, especially for the older kid.
One teenager recently sought help from Reddit regarding his parents' attempts to rebuild their relationship with him. He was primarily raised by his grandparents because his young parents were totally uninterested in taking care of him when he was young. Now that they have two toddlers (the teen in question is 17), they expect him to be a part of their big, happy family that never particularly included him in the first place.