How much attention we pay to time passing also plays a role; children often have fewer distractions and a greater focus on the immediate moment.
Think about a child waiting for Christmas or their birthday. They are intensely focused on the passing days, counting down, thinking about what's to come. This intense focus on time itself can make it feel stretched out. Adults, however, are often busy with work, responsibilities, and future planning. We're less likely to actively observe the ticking clock. Instead, we're usually trying to get things done, and when we're deeply engaged in tasks, time can seem to vanish without us noticing. Our brains are constantly processing information, but as we age, we become more efficient at filtering out 'unimportant' details. This efficiency can mean we pay less attention to the subtle flow of time, only really noticing when we're trying to meet a deadline or waiting for something important. The less attention we give it, the less 'space' it seems to occupy in our minds.