I've been through this, and now at 36, I've decided that I was being stupid and that running is no less meaningful now than it ever was and in fact is probably more meaningful now as it will be more of a challenge to fight the aging process. Not being able to beat your PRs from your 20's is meaningless. 10 year olds cannot compete against 20 year olds, that doesn't mean that it is meaningless for them to try to do well for a 10 year old. Dogs run approximately twice as fast as humans at every conceivable running event up to marathon, that doesn't mean that humans should stop trying. At every stage of life we are a different human with different limitations and challenging ourselves to reach those physical limits is what makes running meaningful. Not only that, the health of our bodies thrive on being challenged. Research has shown that bodies degenerate faster in every way when not being used, including joints, bone mass, muscle fibres, heart, lungs, brain... everything. Fighting against the forces of nature is what literally keeps us alive... so keep up the fight. The entropy of the universe is quite literally fighting against your state of being alive to bring you back to the soil, so keep up that fight until death, because that is what it is to be alive. Do not hang up those spikes. Think about exactly what it is to be young, and exactly what kind of challenges you pursued when you had a youthful mindset and get back into those challenges and continue to pursue them. Aging will inevitably defeat your physical makeup in the end, but do not let aging defeat your spirit.