"“那时的我多么老成，现在的我比那时年轻”"

Simplified Meaning:

When someone says they felt "older then" and "younger now," it means they felt weighed down by serious worries or strict ideas in the past but now feel more relaxed and open-minded. Imagine when you were a teenager, trying hard to appear grown-up, always serious, and following rules closely. Over time, you realized that life doesn't have to be so rigid and serious all the time. As you grew older, you found it okay to have fun, try new things, and not be so stuck on strict beliefs. For example, an adult might have been very focused on career success in their 20s, but in their 40s, they learn to enjoy simple pleasures like hobbies or spending time with friends and family. This shift makes them feel lighter and more youthful. So, the advice is to allow yourself to embrace a more carefree and adventurous mindset, no matter your age.