Does Population Collapse = Economic Collapse?

This might be a dumb question but this thought has came across my mind many times. I’ve watch many videos on YouTube talking about population decline, lower birth rates, and less marriages. Which lead me to consider a future where we have significantly dropped in population within the next 40 years. Will this lower population equate to weaker economic growth and diminishing stock valuations? As someone in their 20s I wonder if this means retirement is unattainable if this trend continues or worsens. Again this may be a senseless concern and I would like to be informed as to why that would be the case. Thanks!