Probably when I was 22 or 23. Around that time a close friend told me that he views his time spent as meaningful as long as he's enjoying himself. I've been going through a lot of hardships and setbacks and still am, so I've often felt depressed, thinking about how I get older and older without much real progress in life, so that view helps me quite a bit, as long as I'm having fun and enjoying my time, I haven't "wasted it". I guess you can always set goals that you want to achieve and you can make accomplishments, but at the end of the day, all of that turns into a memory the second it's gone past, after that, the only thing that is left is how you feel. If you enjoy thinking back and can draw continuous happiness from past achievements, that's great and gives them more meaning (from my perspective), otherwise they'd just be time killed and you could have done litereally anything else, too, unless it of course affects your future in a physical way, too, such as a better job, a relationship, etc.
Here's the twist though, I do think everyone has one general purpose and it is meaningful, which is to leave something behind for future generations. You're born into a family with infrastructure, set up by your peers and are able to enjoy your life thanks to their prior and continuous work, I think it's only fair to not waste their efforts and ensure that your future generations (your own offspring maybe even) can have the same, or an even better start into their life than yours.
SQ